The structures of hetero-oligosaccharides obtained by the action of transglycosylation of Arthrobacter sp. K-1 beta-fructofuranosidase, using sucrose as the fructosyl donor, and several mono- and di-sacchrides as the acceptors were investigated. The main transfer products to most reducing mono- and di-sacchrides were non-reducing oligosaccharides with a fructosyl residue linked to a hemiacetal hydroxyl group. In the presence of L-sorbose, the enzyme produced 2-O-beta-D-fructofuranosyl-alpha-L-sorbopyranoside as the major product. With D-galactose or L-arabinose, the enzyme produced not only non-reducing oligosaccharides, but also reducing oligosaccharides, which were identified as 3-O-beta-D-fructofuranosyl-D-galactopyranose and 4-O-beta-D-fructofuranosyl-L-arabinopyranose, respectively. When a non-reducing sugar such as methyl alpha-glucoside was used as an acceptor, the product formed had a fructosyl residue linked at the C6 hydroxyl group.