The purpose of this study was to establish the dynamics and kinetics of the aromatic hydrocarbons breakdown in aqueous environment. The investigated compounds were: benzene, toluene, m-xylene, p-xylene and styrene. These substances even in low concentrations threaten surface waters taken for municipal purposes because of their toxicity and also mutagenic and tumorigenic properties. The study was carried out under model conditions simulating the environment of river water moderately contaminated and laboratory water ecosystem. The obtained results indicate that the concentration of aromatic hydrocarbons, river microflora adapted to the environment of aromatic hydrocarbons as well as decreased temperature affected the process of the aromatic hydrocarbons' degradation. Due to the significant evaporation, the studied compounds are present in river water only for a short period of time. The half-times calculated in accordance with the kinetic equation of 1st order amounted to 1.1- 8.7 days; the contribution of evaporation was on average 28%. When the microflora had been adapted to the presence of these compounds, the process of degradation was as rule more rapid for both investigated temperatures (20 degrees C and 8 degrees C) and for most of the series. This process was slowed down by temperature fall to about 8 degrees C, but only in the range of low concentrations (1-32 mg/l). This regularity was not observed for higher concentrations up to 146 mg/l. The mixture of aromatic hydrocarbons at concentrations of 3.14-37.11 mg/l, exposed for 20 days to water ecosystem under dynamic conditions was reduced up to 99%. The investigated aromatic hydrocarbons were not absorbed by Elodea canadesis and Physa fortinalis and were weakly condensed in Lebistes reticulatis (cumulation coefficients-0.5-22.8). In the case of bottom sediments, it was stated that these compounds were not cumulated. The investigated concentrations of aromatic hydrocarbons didn't have a positive or negative effect on the aquatic biocenosis settled in the ecosystem.