The pertraction of benzoic acid through an agitated bulk liquid membrane was investigated. The mathematical model based on the diffusion boundary layers was applied for the description of the transport kinetics. The partition of benzoic acid between aqueous and organic phases in the proposed model was described by an approximate linear concentration dependence and by the more exact expression, which takes into account dissociation (K-a) and dimerization (K-2) constants. The effect of the organic solvent on pertraction and model parameters was investigated. It was found that the membrane mass transfer coefficient depends roughly linearly on the reciprocal viscosity of the organic solvent. The model quantitatively described the benzoic acid pertraction, if the parameters characterizing the acid partition were adjusted. If, however, the literature values of K-p and K-2 are applied, the satisfactory model fit was obtained only for cyclohexane used as a liquid membrane.