The aim of this study is the evaluation of the diffusion coefficient of glucose through Saccharomyces cerevisiae biofilms, with thickness ranging from 0.02 to 1.60 mm, in order to carry out the macrokinetic study of alcoholic fermentations in immobilized cell bioreactors provided with matrices with varying porosity. Effective average diffusivities of 1.39 x 10(-6) and 1.44 x 10(-6) cm(2) s(-1) have been calculated at 18 and 30 degrees C, respectively, with no appreciable dependence on biofilm thickness. These values are about 20% of those calculated in water at the same temperatures. The glucose diffusion coefficient through the biofilm at 30 degrees C has been used to carry out a comparison between diffusion, convection and bioreaction mass velocities along a fixed-bed column fed with starch hydrolysate solutions. Although diffusion through the biofilm is the limiting step, biomass grows so abundantly within the support pores at high residence time that the most superficial active layers of biofilm are enough to transform nearly completely the substrate fed. At low residence time the system is not able to stand an evident situation of substrate overloading.