The role of water hyacinth on removal of organic carbon was investigated in free water constructed wetlands. A model incorporating the activities of suspended and biofilm biomasses was developed in order to simulate the various processes involved in the transformation and removal organic matter in the water hyacinths constructed wetland. The results show that the major processes governing the organic carbon transformation and removal in a water hyacinth constructed wetlands system are sedimentation of solids (56.5%), regeneration of organic carbon (25.5%), oxidation of organic carbon to carbon dioxide (6.4%), plant decay (4.4%) and uptake of organic carbon by heterotrophic bacteria (4.2%). The total permanent removal of organic material was 26.7% of the total influent chemical oxygen demand (COD). The COD removal efficiency of the model when the effect of biofilm was considered was 40.5%. However, in absence of biofilm activities, only 34.0% of COD was removed. This confirms the significance of the water hyacinth roots as an attachment media, which is extremely biologically active in assisting the organic carbon removal in the water hyacinth constructed wetland.