This paper investigates the diffusivity of sugars (glucose and xylose) and alcohols (ethanol and xylitol) in a composite immobilized-cell structure that consists of a flat agar layer encompassed between two microporous membrane filters. Viable cells of glucose- and xylose-fermenting yeasts (i.e., Saccharomyces cerevisiae and Candida shehatae, respectively) were entrapped in the gel layer. The diffusional resistance of the composite structure to nonmetabolizable solutes (L-sugar isomers and alcohols) was measured before and after incubation in a two-chambered bioreactor with asymmetrical feeding of the immobilized cell layer. One chamber of the bioreactor was filled with a mixture of glucose and xylose, and the other chamber with a substrate-free medium.