Introduction: Most of the contemporary orthodontic adhesive systems used for bonding are resin-based composite materials. Light-curing systems are predominantly used because of their ability to rapidly achieve high bond strength, ease of handling, and favorable aesthetic characteristics. During light-curing, monomers are converted into polymer, but due to the increase in viscosity of reactive medium complete conversion is unattainable. The degree of conversion (DC) measures the extent of the polymerization reaction, and it is related to mechanical and aesthetic properties as well as the biocompatibility of the material. The conversion of monomers into polymer does not cease when light-curing concludes, rather it is continued until high viscosity completely immobilizes reactive species such as free radicals and monomers, progressively enhancing the DC over time. This phenomenon, known as postcure development of the DC can extend for up to a month following initial reaction. Additionally, in clinical settings the presence of an orthodontic bracket can impede the passage of curing-light (metal bracket) or light can be scattered (ceramic brackets) potentially affecting the material's DC. Literature indicates that the presence of the metal bracket can reduce DC values by 17% to 29% compared to the control group (without bracket). In addition to lightcuring systems, dual-curing systems are also available for clinical application. One of the features of these systems is light-independent, chemically initiated polymerization compensating for insufficient light exposure in areas covered by opaque bracket materials. While numerous studies have investigated the post-cure DC development of resin composite materials used in restorative dentistry, the DC of orthodontic adhesive systems reported in the literature varies significantly, and studies of post-cure DC development of these systems are scarce, especially as a function of bracket materials. Furthermore, most available studies lack a comparison between the DC values obtained in the presence of brackets and the negative control group (without brackets). Hence, the aim of this in vitro study was to investigate how the type of orthodontic bracket material influences polymerization kinetics, DC, and post-cure DC development of resin-based orthodontic adhesive systems. Materials and methods: Five commercially available materials (Enlight (Ormco), Transbond LV (3M), Transbond XT (3M), Heliosit (Ivoclar Vivadent) and Phase II Dual Cure (Reliance)) with different compositions and curing modes (light-curable or dual-curable) were tested under three different light-curing conditions: without brackets (control group, CO), with metal brackets (MB group), and with ceramic brackets (CB group). Samples were light-cured using the LED curing unit with a continuous intensity of 1000 mW/cm2 (Bluephase G2, Ivoclar Vivadent) for 20 s positioned directly above material or CB, and 10 s mesially and 10 s distally for samples with MB. In total 450 samples were tested. To determine the polymerization kinetics, DC, and post-cure DC development, Fourier transform infrared spectroscopy (FTIR) with attenuated total reflectance (ATR) was used. FTIR spectra were collected in the 500 - 3500 cm-1 wave number range at a spectral resolution of 8 cm-1 for polymerization kinetics measurements and 4 cm-1 for post-cure DC measurements. Real-time measurements (n = 6 per each bracket and material combination) were performed to determine short-term DC in 2, 6, and 10 minutes of measurement. Each spectrum was recorded in two scans, at a data collection rate of two spectra per second, with continuous spectral recording for 10 minutes from the activation of the light-curing unit. In total 90 samples were tested. To determine post-cure DC values, measurements were performed 1, 7, and 28 days on previously prepared and polymerized samples (n = 8 per each bracket and material combination, and time point) that were withdrawn from the laboratory incubator where they were stored in dry conditions at 37 0C. One spectrum was recorded at each time point using 30 scans per spectrum. In total, 360 samples were tested. The analysis of the FTIR spectra and the DC calculations were performed according to Rueggeberg’s standard baseline method. Statistical analysis was performed using one-way ANOVA with Tukey post-hoc correction at an overall significance level of 0.05. Results: Short-term DC values obtained from polymerization kinetics measurements, ranged from 43.9 % to 76.1 %. The MB group had significantly lower DC values than the CO group (up to 14.8 %) and CB group (up to 13.3 %). Most of the materials in the CB group had statistically similar values to the CO group, except Enlight and Phase II Dual Cure where DC values were slightly lower (up to 1.5 %). Phase II Dual Cure obtained the highest DC values throughout 10 minutes of measurement in all groups, with DC values from 67.8 % (MB group) to 76.1 % (CO group) at the end of the measurement. Transbond XT obtained the lowest DC values in the CO and CB group throughout the measurement with a DC value of 53.5 % in the CO and 53.3 % in the CB group at the end of the measurement. In the MB group, Enlight obtained the lowest DC values with a DC of 49.7 % at the end of the measurement. The long-term DC values were higher than the short-term, ranging from 54.3 % to 85.3 %. Seven days after light-curing, some of the materials showed statistically higher DC values compared to the 1-day-after light-curing (Heliost in the CO (1.7 %) and the CB (1.0 %) group and Phase II Dual Cure (2.8 %) in the CO group). In the MB group, Transbond LV, Heliosit, and Phase II Dual Cured had statistically lower (1.3% to 3.9%) DC values compared to the CO group. Most of the materials and groups showed a statistically significant increase (0.7 % – 4.8 %) in the DC values between 1 and 28 days after light-curing, except Transbond LV and Phase II Dual Cure, with the increase of the DC values only in the CO group. In the MB group, DC values were significantly lower compared to the CO (2.0 % – 4.3 %) and the CB group (0.8 % – 2.2 %) for Enlight, Transbond LV, and Heliosit. Enlight and Transbond XT showed a substantial rise (19.2 % – 32.9 %) in the DC values obtained at the end of polymerization kinetics measurement and 1 day after light-curing in all groups. Conclusion: These findings suggest that the polymerization kinetics, DC, and post-cure development of the DC of orthodontic adhesive systems are highly dependent on the materials used and are influenced by the type of bracket material present. The presence of metal brackets has a more pronounced negative impact on both short- and long-term DC values compared to ceramic brackets. However, this effect diminishes over time due to post-cure polymerization.