The permeation of lactic acid from aqueous solutions through a supported liquid membrane containing tri-n-octylamine as mobile carrier was studied experimentally and theoretically. The strip phase was either Na2CO3 solution or deionized water. The membrane support was Durapore hydrophobic GVHP film. A permeation model was proposed considering aqueous film diffusion, interfacial chemical reaction, and membrane diffusion. The calculated rates were found to agree reasonably with the measured ones (average standard error, 13%). Under the ranges investigated, it was shown that the present permeation process was governed by mixed interfacial reaction and membrane diffusion.