Glucose conversion into disaccharides was performed with beta-glucosidases from Prunus dulcis (beta-Pd), Aspergillus niger (beta-An) and A. awamori (beta-Aa), in reactions containing initial glucose of 700 and 900 g l(-1). The reactions' time courses were followed regarding glucose and product concentrations. In all cases, there was a predominant formation of gentiobiose over cellobiose and also of oligosaccharides with a higher molecular mass. For reactions containing 700 g glucose l(-1), the final substrate conversions were 33, 38, and 23.5% for beta-An, beta-Aa, and beta-Pd, respectively. The use of beta-An yielded 103 g gentiobiose l(-1) (15.5% yield), which is the highest reported for a fungal beta-glucosidase. The increase in glucose concentration to 900 g l(-1) resulted in a significant increase in disaccharide synthesis by beta-Pd, reaching 128 g gentiobiose l(-1) (15% yield), while for beta-An and beta-Aa, there was a shift toward the synthesis of higher oligosaccharides. beta-Pd and the fungal beta-An and beta-Aa beta-glucosidases present quite dissimilar kinetics and selective properties regarding the synthesis of disaccharides; while beta-Pd showed the highest productivity for gentiobiose synthesis, beta-An presented the highest specificity.