Enzymatic depolymerization of polysaccharides with a-amylase has been studied in mixed aqueous dimethylsulfoxide (DMSO)/water solvents. Polysaccharide substrate chemical compositions, configurational structures, and bonding patterns are known to affect observed enzymatic reaction kinetics. Tt-le branching structures of polysaccharides and their effects on the kinetic mechanisms of depolymerization reactions via endo-acting hydrolyzing enzyme was studied via size exclusion chromatography coupled to low angle laser light scattering (SEC/LALLS). The glycogen branching structure is a heterogeneously distributed "cluster" structure rather than a homogeneously distributed "treelike" structure. The action pattern of alpha-amylase on glycogen, which is composed of highly branched clusters, as end-products, has a "pseudo-exo-attack" in contrast to an expected "endo-attack" as seen in the hydrolysis of amylose or amylopectin substrates. These effects df branched substrates for mixed amylose/glycogen alpha-amylolysis have been predicted and demonstrated by both experimental and theoretical analysis using the kinetic model presented in this report. The "lumped" kinetic model employed, assumes that the enzyme simultaneously attacks both linear and branched substrates. In general, excellent agreement between the model predictions and the experimental observations, both qualitatively and quantitatively, was obtained.