We have made an experimental study of the isomerization of fructose and glucose by glucose isomerase in two forms : one soluble and the other immobilized by being fixed in a gel matrix. For the latter technique we used a fixed-bed differential recirculation reactor, which allowed us to obtain comparable results to those we got with the enzyme dissolved within a batch reactor. We propose a simple method of comparing the results obtained in both systems, which permits us to determine the relative activity of each enzyme and also the effective diffusivity in the immobilized enzyme. The values obtained for the latter indicate a tortuosity factor of 2.04, which is lower than that generally found in porous solids but consistent with the results published in the literature for diffusion in gel particles. The application of our method to the results arrived at with the two systems studied confirms the validity of the kinetic model for glucose-fructose isomerization and has lead to our being able to determine both the kinetic constants for the reaction and their variation according to temperature.