Although trace metals are present in the environment in small, but measurable quantities, they are non-deagradable and their accumulation can produce negative effects on the environment. In this graduate thesis, type of microplastics in sediment samples was determined, and after that, both the amounts of Zn, Cd, Pb and Cu adsorbed on microplastics surface, and their concentrations in seawater were measured. Microplastic particles were analysed using Fourier-transform infrared spectroscopy method in ATR mode. From 30 samples that were analysed using this technique, 23 samples gave spectrums that were comparable with spectrums from the data-base, and from those 23 samples, 10 of them showed spectrum characteristic for polystyrene (PS), 10 for polypropylene (PP), 2 for high-density polyethylene (HD-PE) and 1 for low-density polyethylene (LD-PE). Analytical method used for measuring amounts of metals was differential pulse anodic stripping voltammetry. Considering all measured metals, Zn showed the highest adsorption capacity, while Cd demonstrated the lowest adsorption capacity on microplastics surfaces. Except Cd, metals showed higher adsorption capacity for samples that contained only polystyrene microplastics particles in regards to samples that contained mixed microplastics. Mass fractions of trace metals that adsorb on microplastics surface were in the range of metals mass fractions in unpolluted and moderately affected sediments of the Croatian Adriatic Sea. The mass fractions of Zn, Pb and Cu increased with decreasing plastic particle size. Exception is fraction larger than 4 mm, both mixed and polystyrene samples, where the amounts of metals were higher compared to particles of 2-4 mm and 1-2 mm. Concentrations of all trace metals (both dissolved and total) in seawater at Mala Martinska location were significantly higher compared to the control sample from Jadrija sampling point.