Humic acid is a major component of natural organic matter in surface water and can cause serious fouling problems in membrane filtration processes for drinking water treatment. The transport of humic acid to the membrane surface and within membrane pores is related to its diffusivity. Thus. the diffusion coefficient of humic acid is an important mass transport parameter. Clark and Lucas [J. Membr. Sci. 143 (1998) 13-25] studied the diffusion and partitioning of humic acid into a porous ultrafiltration membrane and developed a model to predict how humic acid diffusivity changes under different pH and calcium concentration conditions. In this work, the diffusion coefficient of humic acid was directly measured and compared to the predictions of the Clark and Lucas model. The experiments were conducted in a two-chamber diffusion cell separated by a track-etched membrane. The results show that the diffusivity of humic acid increases with decreasing pH and increasing calcium concentration, which can be explained by the compaction of humic acid molecules at low pH and high ionic strength. The experimental measurements strongly support the predictions of the Clark and Lucas model.