The objective of the present work was to investigate the mass transport and fouling mechanism of direct contact membrane distillation (DCMD) process. The experiments were performed on a flat sheet module using pure water and humic acid solution as feeds. The membrane employed was hydrophobic PVDF of pore size 0.22 mu m. The mass transfer models based on Dusty gas model were applied to tit the flux data and the pressure blocking filtration laws were employed to explain the membrane fouling. The results showed that molecular diffusion was the most suitable model for predicting fluxes of both laminar and turbulent flow. Fouling of the membrane by humic acid aggregates can be described by cake filtration model. The transport resistance of the feed boundary layer was higher than other resistances, and fouling resistance increased significantly with time. (c) 2005 Elsevier B.V. All rights reserved.