Continuous leucrose formation with dextransucrase entrapped in calcium alginate beads was investigated A plug-flow packed bed reactor was used as the reaction vessel and the conversion of sucrose as a function of residence time was measured at different initial sucrose concentrations. Leucrose productivity by the immobilized enzyme at different reaction conditions was determined and compared to the productivity of a discontinuous operation process. Productivities of the continuous process were in a range of 0.7-3.6 g U-1, whereas only 0.7 g U-1 could be reached in the discontinuous process; moreover, inactivation constants of the immobilized enzyme in the continuous operation mode were determined. The enzyme was continually inactivated with increasing reaction time. This was due to transport limitations inside the enzyme particles that could be confirmed by investigation of the swelling behavior of the enzyme beads due to the formation of dextran.