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Aquatic humic and fulvic acids can increase the permeability of biological membranes to lipophilic solutes. In in vivo experiments, passive diffusion of fluorescein diacetate (FDA) into the green alga Selenastrum capricornutum increased in the presence of Suwannee River humic and fulvic acids at pH 5 (humic > fulvic) but not at pH 7. The observation of enhanced diffusion at the lower pH is consistent with adsorption measurements, which showed that the association of humic and fulvic acids with the algal surface was greater at pH 5 than at pH 7. Permeability experiments were also performed on model membranes to investigate the interaction of these humic substances with membrane lipids. In these in vitro experiments, we followed leakage of the fluorescent probe sulforhodamine-B (SRB) that had been encapsulated within 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) vesicles; this model phospholipid is representative of those found in the plasmalemma of green algae. Release of SRB from the vesicles was markedly accelerated in the presence of Suwannee River humic and fulvic acids (humic > fulvic); for the humic acid, lowering the pH from 7.6 to 5.7 enhanced this surfactant-like effect. The demonstration that humic substances can alter the permeability of phytoplankton and model membranes at natural concentrations and pH values has potential implications for the uptake and regulation of toxic and essential solutes by the phytoplankton community.