Control of the swelling of two different liquid emulsion membrane systems for separation of lactic acid was examined. The major disadvantages of swelling are dilution of the separated product and emulsion breakage. Several additives including liquid paraffin, cyclohexanone and n-decanol were investigated with respect to both emulsion swelling and lactic acid separation rate. If swelling is a function of surfactant concentration, the swelling increases with the quantity of the hydrophilic part in the surfactant. Therefore, a surfactant with a low hydrophilic/lipophilic balance (HLB) value should be used to reduce the swelling. The use of Span 85 (HLB = 1.8) as co-surfactant achieved the objective in the case of a tri-n-octylamine/Span 80 system.