The operation of three hybrid constructed wetland systems composed with vertical flow (VF-CW) and horizontal flow (HF-CW) constructed wetlands was analysed. The analyses were carried out in two wetlands (CWs) located in northern Poland: in Wiklino and Wieszyno and one in Wiedersberg in Germany. The wetlands were supplied with domestic sewage after mechanical treatment. It was proved that the decomposition rate of organic matter was determined by proportions of organic hardly decomposed (COD-X-S) and non-decomposed (COD-X-1) suspension. It was proved that increase of organic matter (OM) concentration in filter material of first beds in plants: Wiklino and Wieszyno caused clogging and decreased removal efficiency. In the two-years long study no accumulation of organic matter in the VF-CW in Wiedersberg was observed. Therefore, the aerobic conditions maintained in the bed enabled efficient decomposition of soluble organic easy-to-decompose fraction (COD-S-S)and COD-X-S and no accumulation of OM was observed. The effectiveness of COD removal in the VF-CW in Wiklino and Wieszyno was lower in comparison to Wiedersberg, which was due to lower biodegradation potential of treated sewage (in Wiklino) and accumulation of OM in the sub-surface bed layer, resulting in pores clogging and reduction of air inflow (in Wieszyno). (C) 2007 Elsevier Ltd. All rights reserved.