To identify the parameters that affect enzymatic hydrolysis at high substrate concentrations, whey protein isolate (1-30% w/v) was hydrolyzed by Alcalase and Neutrase at constant enzyme-to-substrate ratio. No changes were observed in the solubility and the aggregation state of the proteins. With increasing concentration, both the hydrolysis rate and the final DH decreased, from 0.14 to 0.015 s(-1) and from 24 to 15%, respectively. The presence of 0.5 M NaCl decreased the rate of hydrolysis for low concentrations (to 0.018 s(-1) for 1% WPI), resulting in similar rates of hydrolysis for all substrate concentrations. The conductivity increase (by increasing the protein concentration, or by addition of NaCl) has significant effects on the hydrolysis kinetics, but the reason for this is not yet well understood. The results show the importance of conductivity as a factor that influences the kinetics of the hydrolysis, as well as the composition of the hydrolysates.