In this research the influence of substrate structure on the kinetics of the enzymatic hydrolysis of starch by glucoamylase was evaluated. For this purpose, two substrates of different form and molecular weight were used. In one case, the kinetics of the hydrolysis corresponds to a typical Michaelis-Menten behavior; in the other, a decrease of the hydrolysis rate occurred once a determined substrate concentration was surpassed. The structural differences between the starches, which caused important differences on the rheological properties of their solutions, justify the observed differences in their behavior. Branching of the substrate exerts two opposite effects on the hydrolysis rate because it allows the increase of the number of available points for the enzymatic attack, although the branching increases the steric hindrances and, consequently, the mass transfer resistances. The balance between these two effects is dearly dependent on the substrate concentration.