The increment of activity and solubility in 1,4-dioxane of lipase B from Candida antarctica, lipase from Pseudomonas cepacia, and subtilisin, were investigated as a function of the methoxypoly(ethylene glycol)–protein (PEG–protein) ratio employed during lyophilization. Both activity and solubility markedly increased as the PEG–protein ratio was increased. The increment of activity at low PEG–protein ratios, however, was much higher than that of solubility. These data suggest that the PEG-induced activation effect is due mainly to a lyoprotection effect rather than to relaxation of diffusional limitations.Key words: hydrolases, activity, dispersion, lyoprotection, circular dichroism.