Conversion of fumaric acid (FA) to L-malic acid (LMA) was carried out in a bioreactor divided by two supported liquid membranes (SLMs) into three compartments: Feed, Reaction, and Product. The Feed/Reaction SLM, made of tri-n-octylphosphine oxide (vol 10%) in ethyl acetate, was selective toward the substrate, fumaric acid (SFA/LMA = 10). The Reaction/Product SLM, made of di(2-ethylhexyl) phosphate (vol 10%) in dichloromethane, was selective toward the product, L-malic acid (SLMA/FA = 680). Immobilized yeast engineered to overproduce the enzyme fumarase [E.C. 220.127.116.11] was placed in the Reaction compartment and served as the catalyst. The yeast was immobilized in small glasslike beads of alginate-silicate sol-gel matrix. The construction of the bioreactor ensured unidirectional flow of the substrate from the Feed to the Reaction and of the product from the Reaction to the Product compartments, with the inorganic counterion traveling in the opposite direction. The conversion of almost 100%, above the equilibrium value of ca. 84% and higher than that for the industrial process, 70%, was achieved. In contrast to the existing industrial biocatalytic process resulting in L-malic acid salts, direct production of the free acid is described.