A method of determining of the effective diffusion coefficient of substrate in a particle, where the diffusion and consumption of substrate by biocatalytic reaction are present simultaneously, was designed and experimentally verified. The method is based on measuring the overall rate of heterogeneous biocatalytic reaction in particles of varying diameter The effective diffusion coefficient D-e, was determined by fitting the measured reaction rates with the solution of the reaction-diffusion equation. The method is tailored for cases where the enzyme reaction is governed by the Michaelis-Menten kinetics. The value of K-m required for the solution of the mathematical model was adopted from the measurement of the kinetics of free cells, whereas the rate parameter, k(2), was optimized together with D-e. As an experimental model, the sucrose hydrolysis catalyzed by Ca-alginate-entrapped yeast cells was examined. The particle diameter varied in the range of 1.2-3.9 mm and the initial reaction rates were measured in a batch-stirred reactor at a sucrose concentration of 100 mM. The D-e of sucrose at 30 degrees C was found to be 2.9 . 10(-10) m(2)s(-1).