Galactooligosaccharides (GalOS) production from a lactose solution (16% w/v) was studied at 70 degrees C, pH 4.6 and 6.0 with free or immobilized beta-galactosidases of Thermus aquaticus YT-1. Synthesis of OS and lactose conversion was significantly improved when using the immobilized preparation. At pH 4.6, OS accounted for approx. 35% of the total carbohydrates with a conversion rate of lactose of 80%. The general formula of OS synthesized in this way was Gal-(Gal)n-Glu with n being equal to 1 or 2 and a beta anomeric configuration of the D-galactosyl moiety.