In the enzymatic synthesis of cefaclor, 3-chloro-7-d-(2-phenylglycinamide)-3-cephem-4-carboxylic acid, from phenylglycine methyl ester and 7-aminodesacetoxymethyl-3-chlorocephalosporanic acid, the in situ product could influence both the overall conversion and hydrolysis of the ester. Optimization of the parameters, such as pH 6.2, 5 degreesC and substrate molar ratio of 2:1, made in situ product removal improve the overall conversion from 64% to 85% (mol/mol).