The enzymatic depolymerization of polygalacturonic acid was studied in a continuous stirred UF membrane reactor. Experimental data were obtained in different runs with varying amounts of biocatalyst from 0.25 to 1.35 mg. The residence time (1.7 h), temperature (25 °C), and membrane molecular weight cutoff (30 kDa) were kept constant. The time course of the oligomeric product concentration in the output solution was correlated with a model equation, that was originally developed for batch reactions and modified for the continuous system. A new experimental method was used to detect the instantaneous mean molecular weight. Evidence was obtained that the enzymatic breakdown was almost complete within 2−3 h from the startup of the bioreaction. The proposed method proved to be sufficiently accurate and easy to employ for rapid on-line control of the industrial bioprocess. Copyright © 2003 American Chemical Society