An experimental device that quantifies the diffusional properties of polysaccharides solution, in a simple and reproducible way, and an equation relating these properties to solute concentration, is proposed. Using this model to analyse the kinetic profile of the hydrolysis of starch with glucoamylase, is substrate inhibition completely reduced to phenomena that depends on the reological properties of the reacting mixture. The experimental device and kinetic model can be applied to contrast hypothetical substrate inhibitions in any enzymatic system.