The addition of ethylene glycol, and 1,2- and 1,3-propanediol, decreases the bulk dielectric constant of the medium, and according to CD measurements, increases significantly the proportion of helical structure in beta-lactoglobulin. The medium-induced folding changes followed by limited peptic hydrolysis show that the cleavage of beta-lactoglobulin by pepsin is triggered by structural transformations induced by ethylene glycol only and not by 1, 2- and 1, 3-propanediol. Density measurements, at constant chemical potential and constant molality, demonstrate that all diols are present in the immediate domain of the protein. They are engaged in hydrophobic interactions with the amino acids of beta-lactoglobulin core inducing the formation of additional alpha-helices.