The transport of citric acid from aqueous solutions through a supported liquid membrane containing tri-n-octylamine was studied. The strip phase was either a sodium carbonate solution or deionized water. A transport model was proposed considering aqueous film diffusion, interfacial chemical reaction, and membrane diffusion. The calculated rates were found to be in good agreement with the measured ones (average standard error, 12%). Under the conditions examined, it was shown that the transport process was governed by mixed interfacial chemical reaction and membrane diffusion. The effects of temperature and a water-insoluble organic acid added in the membrane phase on transport rate were also investigated.