A study has been made of the kinetics of fructose-glucose enzymatic isomerization with an immobilized glucose isomerase, Sweetzyme A, at 60 degrees C and with initial fructose concentrations of between 0.1 and 2.0 M. In the kinetic study, the effects of internal and external transport were considered, and it was found that when the data were fitted to a kinetic of a pseudo-first-order type, the non-enzymatic reaction could not be disregarded at low enzyme and high substrate concentrations. When non-enzymatic isomerization is taken into account, the true enzymatic-reaction constant can be calculated, giving values for maximum rate of 1.39.10(-6) mol (kg.s)(-1) and for Michaelis-Menten constant of 1.24 M. Establishing the influence of the transport in the interior of the catalyzing particles has enabled the determination of a value for effective diffusivity of 0.59.10(-10) m(2).s(-1).