The pollution of the sea by plastic waste is one of the most urgent global problems of the marine environment all over the world. It has attracted worldwide attention from governments, the public, the scientific community, the media and non-governmental organizations and has become a hot topic in current marine ecology and environmental research. The aim of this work was to investigate the influence of temperature and age of poly(ethylene-terephthalate) (PET) microplastics on the adsorption of the pesticide simazine (SMZ). Microplastic particles (10-100 μm) prepared from pristine PET films and films subjected to accelerated aging were used 56 days under UV radiation were used. After grinding in a cryomill, the particle size distribution of microplastics was determined, using a vibrating shaker with sieves and a laboratory sieve with an air jet were used to separate the fractions. Adsorption experiments were carried out using a thermostated laboratory shaker, and changes in simazine concentration were monitored by high-performance liquid chromatography (HPLC). In the end, the individual harmful effects of microplastics and simazine and the overall effects of microplastics with adsorbed pollutant were tested by performing standard acute toxicity tests on the green microalgae Pseudokirchneriella subcapitata (ISO 8692). The results of ATR-FTIR spectroscopy showed that UV aging increases the proportion of smaller fractions of microplastic particles, which means that due to the influence of aging, the material is more susceptible to fragmentation, and oxidative decomposition has occurred, which results in the formation of functional groups containing oxygen (reduced hydrophobicity of the material). The results of simazine adsorption follow the Langmuir model, which indicates adsorption in a monolayer and the distribution of active sites on the adsorbent is homogeneous. Adsorption is more pronounced in the case of aged samples compared to pristine microplastic samples, and the highest simazine adsorption was achieved with PET 56 at a temperature of 20°C. The toxicity test on the green microalgae Pseudokirchneriella subcapitata showed the highest toxicity with the combined samples of simazine with aged microplastic (PET 56) and CaCl2.