Enzymic reaction of lactase was carried out to produce galacto-oligosaccharide with a high degree of conversion. The reaction process was analysed with a simple model. The reaction involved a batch process operated with initial lactose concentrations ranging from 0.139 to 1.67 mol litre(-1) at 40 degrees C and pH 4.5 with beta-galactosidase from Aspergillus oryzae. A maximum oligosaccharide conversion of over 30% was obtained with a lactose feed concentration greater than I II mol litre(-1). The production process was mathematically modelled from the basic material balance and the rate equation and showed agreement between simulated and experimental results. The rate constants of the reaction strongly correlated with the viscosity which ranged from 0.9 to 6.6 mPa in the feed solution.