Enzyme membrane reactors combined with the large interfacial area of a fine emulsion, called emulsion enzyme membrane reactors (E-EMRs), were investigated for the optical resolution of (R,S)-naproxen methyl esters to produce (S)-naproxen. Preparation of a fine emulsion by continuous premix membrane emulsification (ME) was studied and compared to direct ME. By continuous ME, an emulsion with 1.32 mu m of average droplet size and 46.5% (v/v) of organic phase content was obtained using a Shirasu porous glass (SPG) membrane of 5 mu m pore size under 0.12 MPa pressure. The performance of the E-EMR, including mass transfer, reaction rate, enzyme activity, productivity, enantiomeric excess and enantioselectivity, were studied with different loads of lipase on the membrane or with different substrate concentrations in the emulsion. The immobilized enzyme layer caused the main mass transfer resistance in E-EMRs. An increase of the amount of lipase in the membrane did not result in a production increase but an increase in substrate concentration led to an increase in reaction rate. Up to 100% of enantiomeric excess and more than 100 of enantioselectivity was obtained in E-EMR. (C) 2008 Elsevier B.V. All rights reserved.