Mechanisms of proton transport were investigated in phospholipid bilayer membranes exposed to salicylates and benzoates. Membranes were formed from diphytanoyl phosphatidylcholine in decane plus chlorodecane (50% vol/vol). Proton and anion conductances (GH and GA) were calculated from the total conductances and the H+ or A diffusion potentials produced by transmembrane H+ or A gradients. At low pH salicylate caused a GH which was proportional to the square of the total weak acid concentration, and GH was maximum when pH = pK. At neutral to alkaline pH salicylate caused a GA which was proportional to the first power of the salicylate concentration, and GA was independent of pH. Both GH and GA were inhibited by phloretin. The results suggest that salicylate acts as an HA2-type proton carrier at low pH and as a lipid-soluble anion at neutral pH. Salicylate has been implicated as a causal factor in Reye's syndrome, as well as in aspirin poisoning, and salicylate has been reported to increase the proton conductance of inner mitochondrial membranes. The present results suggest that in mitochondria salicylate increases passive proton uptake by a combination of HA influx (driven by the concentration gradient) and A efflux (driven by the voltage and concentration gradients). Model calculations suggest that over the range of therapeutic to toxic concentrations, salicylate causes net H+ influx sufficient to explain the reported "loose coupling," uncoupling and swelling of mitochondria. The relative ineffectiveness of aspirin and benzoate can be explained by their low A permeabilities, whereas the ineffectiveness of 2,6-dihydroxybenzoate can be explained by its low pK.