Beta-fructofuranosidase fructosylated not only the hydroxyl group but also the thiol group of 2-mercaptoethanol in a transfer reaction using sucrose as a donor substrate. The enzymes from Candida utilis and Saccharomyces cerevisiae (bakers' yeast) were effective catalysts for the thio-fructofuranosylation. The thio-fructosylation product was isolated by activated carbon chromatography and its structure was confirmed by Fab-mass spectrometry and NMR spectroscopy. The thio-fructofuranoside was synthesized effectively at around 3.0 M for the acceptor concentration. The product increased with the sucrose concentration at least up to 1.9 M. O-Fructofuranoside was simultaneously synthesized at an early stage of the reaction, although it was hydrolyzed on further incubation. On the contrary, the thio-fructofuranoside accumulated efficiently after synthesis, indicating it was very stable against the hydrolytic action of the beta-fructofranosidase.