Studies have been made on transport rates and separation of propionic over acetic acid by the solution-diffusion type pertraction in a multimembrane hybrid system (MHS). The membrane system has been composed of two hydrophilic, strongly basic or acidic, polymer membranes separated by a hydrophobic bulk liquid membrane. Separation of propionic and acetic acids from their aqueous solutions (0.1–0.8 M) or real fermentation broth has been studied. Experimentally observed mass transfer coefficients (k) range from 3.5×10⁻⁶ to 1.5×10⁻⁵ cm/s with the higher values for propionic acid. In the case of fermented broth the fluxes were found to depend on its pH and the pretreatment of the feed. The selectivity SEL_AA^PA evaluated as the ratio of propionic and acetic acid fluxes can attain values ranging from 3.5 to 5.5. The overall transport mechanism is consistent with a solution-diffusion mechanism typical for liquid membranes operating without a carrier. The fluxes and selectivity of the membrane system in respect to acetic and propionic acid have been found to depend on the viscosity of liquid membrane phase and solubility parameters characterizing both acids and solvents. A model based on fundamental physicochemical properties of the permeants and solvents (diffusion coefficients, solubility parameters and molar volumes) has been elaborated, and experimentally verified.