Plastics is an inevitable part of our daily life, hence its usage will continue to grow. Multiple and prolonged use and abrasion of the plastic surface might result in partial deterioration of plastic to micro-sized particles – microplastic (MP). Recently, it has been observed that MP’s multi-faceted structural and surface properties makes it an adsorbent for numerous aquatic pollutants such as pharmaceuticals and pesticides, defined under the EU water protection legislation as contaminants of emerging concern (CECs). Their hydrophobicity, polarity and low-biodegradability makes them readily adsorbable at polar, hydrophobic, aged and degraded specific surfaces of MP, contributing to the overall toxicity of such binary systems. Besides, in environmental conditions MP might leach additives and constituents that are often found toxic and hazard. The aim of this dissertation was to investigate the influence of thermal- and photo-aging of polyethylene terephthalate (PET) and polypropylene (PP) on the changes in morphology and structure of polymer materials and the overall ecotoxicity of PET and PP MPs in water. The characterization of pristine and aged MPs was performed by Fourier-transform infrared spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), as well as Water Contact Angle (goniometer) and the Branuer Emmett Teller (BET) specific surface area measurements. In order to investigate the fragmentation of plastic materials into MPs, grinding was performed in a single-drum cryogenic mill, while size distribution of obtained MPs particles was determined by the sieve shaker with five sieves of different mesh sizes to separate the particles according to their size. Adsorption of selected CECs, pharmaceutical diclofenac (DCF) and pesticide atrazine (ATZ), was investigated on both pristine and aged MPs according to the computational/empirical approach employing Full Factorial Design (FFD) as the experimental plan in the combination with Response Surface Modelling (RSM). Furthermore, pristine and aged MPs along with PP waste packaging films, were submitted to leaching tests in order to study possible migration of consituents. The main focus of this dissertation was the evaluation of the aquatic ecotoxicity of the samples after the adsorption and leaching tests and it was tested on the marine bacteria Vibrio fischeri (VF), freshwater crustacean Daphnia magna (DM) and green microalgae Raphidocelis subcapitata (RS), following the procedures disclosed in ISO 11348-3:2007, ISO 6341:2012 and ISO 8692:2012 standards, respectively. Finally, the leachates were tested for their biodegradability, along with glucose as readiliy biodegradable substrate. Based on the obtained results, it is possible to establish a direct relationship between the aging of MP and the increase in its toxicity. Characterization revealed severe degradation of material; thermal-aging affected the structure, while photo-aging caused the surface alterations of PET and PP. Hence, changes in surface morphology such as cracks, fractures, notches and bumps were observed. The increase in crystallinity of PET and PP resulted in changes of brittleness and accelerated fragmentation to smaller MP’s particles. It was found that hydrophobicity increased, while specific surface decreased; the exception was photo-aged PP. Adsorption of CECs onto MPs is correlated with the changes in the surface morphology enhanced with the photo-aging. Adsorption of CECs was driven by hydrophobicity and specific surface, as well as smaller MP particle sizes as a result of accelerated and prolonged aging period. RSM approach revealed that temperature is a more significant parameter than MP dosage for CECs adsorption. It can be concluded that aquatic ecotoxicity generally increases with the prolonged exposure to aging, especially valid for the photo-aged MPs. However, some exceptions were observed depending on the type of MPs, CECs and targeted toxicity test organism. Individual CECs showed high ecotoxicity effects toward DM, while slightly lower effects were recorded toward RS. It was also found that DM was the most sensitive organism to the exposure to MPs, with and without CECs. The aging of MPs resulted with the accelerated leaching of constituents for both PET and PP, resulting with ecotoxic responses in cases of all three tested organisms. However, DM was shown to be the most sensitive to leachates originated from PP-MP, while in the case of PET leachates other two test organisms (VF and RS) were more sensitive in ecotoxicity response than DM. The biodegradability of glucose was inhibited by the presence of (heavy) metals in leachates originated from both thermal- and photo-aged PET and PP MPs.