The object of our research were vertical flow constructed wetlands (VFCWs) that are a component of biological treatment of two hybrid hydrophyte systems located in Wieszyno and Wiklino, near Słupsk, Poland. The facilities that are subject to analyses are unified in terms of structure (filtration material, depth of bed and time of operation), but they differ in organic matter load, amounting respectively to: 8.0 gCOD/m²day and 31.0 gCOD/m²day. The intensity of oxygenation of the facilities resulting from diffusive flow of oxygen to the soil was determined based on the measurements of oxygen diffusion coefficient in the soil (D_g). The analysis of the quality of organic matter in wastewater was concentrated on determining concentrations of the following four fractions: in suspended solids and in dissolved phase both decomposable and non-decomposable ones. The investigation proved that the increase in load of organic matter resulted in deterioration of oxygen conditions in the bed, which in turn led to a decrease in efficiency of pollutant removal. Higher load of organic matter in the facility of Wieszyno as compared with Wiklino was the direct cause of VFCW clogging.