A diffusion cell was used to experimentally determine the diffusivity of natural organic matter (NOM). Various diffusivities were obtained for NOM with respect to both pH and the molecular weight cutoff (MWCO) of membranes. Values determined with the diffusion cell were compared to those estimated from flow field-flow fractionation (fl-FFF) and high-performance size exclusion (HP-SEC). Firstly, the diffusivities obtained with the diffusion cell were much lower than those from both fl-FFF and HP-SEC,which was due to the interference in NOM transmission through the membrane Pores. Secondly, the diffusivity of NOM increased with decreasing pH due to both electrostatic interactions and double layer compaction. Thus, at lower pH, the mass transfer coefficient of NOM increased due to increases in the corresponding diffusivity. Thirdly, the diffusivity of NOM decreased as the MWCO of a membrane decreased, which was the result of the sieving effect between the NOM and the membrane. Along with the experimental identification of NOM diffusivity, the transport parameters (mass transfer coefficient (k), solute permeability (P-m), and membrane selectivity (sigma)) of NOM under different pH conditions were also investigated using irreversible thermodynamic models. The transport parameters of NOM in a membrane system depended on the permeability and surface charge. (c) 2008 Elsevier B.V. All rights reserved.