An empirical rate equation having two parameters was proposed for the reaction catalyzed by beta-fructofuranosidase (FFase) that was used for the production of fructooligosaccharides. The rate equation enabled the estimation of FFase activity from the concentration of reducing sugar in the reaction product. The rate equation was applied to the case of immobilized FFase in a packed-bed reactor to determine the effects of linear velocity on the performance of the reactor and to monitor the changes in activity during a 10-d operation. It was considered that the proposed rate equation could be a valuable tool for quantitating the effects of various parameters related to the process development of fructooligosaccharides.