Response surface methodology (RSM) was used to optimize the enzymatic synthesis of butylgalactoside from lactose catalyzed by beta-galactosidase from Aspergillus oryzae. The empirical models developed by using RSM were adequate to describe relationships between the operating conditions (temperature, water-to-butanol volume ratio, lactose concentration, enzyme concentration) and the responses (butylgalactoside concentration, conversion yield). Based on contour plots and canonical analysis, optimal conditions for maximizing butylgalactoside concentration were: temperature (45 degrees C), water-to-butanol volume ratio (44%), lactose concentration (134 g/l), and enzyme concentration ( 1.5 g/l). Experimental data indicated that up to 24 g/l were produced at the optimum point Maximum conversion yield of 79.5% was obtained at: temperature (46 degrees C), water-to-butanol volume ratio (18%), lactose concentration (10 g/l), and enzyme concentration (1.5 gill. The models were verified experimentally. Synthesis at a large scale was successful.