Catalytic elongation by dextransucrase (DSase) was monitored directly on a dextran-acceptor- or DSase-immobilized 27 MHz quartz crystal microbalance (QCM) Kinetic parameters for the binding of the enzyme to the dextran acceptor (k(on), k(off), and K-d) and enzymatic elongation in the presence of a sucrose Monomer (K-m for sucrose and k(cat)) were determined. The kinetic parameters obtained by both methods were consistent.