Abstract The kinetics of denaturation of ovalbumin and S-ovalbumin by butyl alcohol isomers (butyl alcohol, isobutyl alcohol, s-butyl alcohol, and t-butyl alcohol) have been studied. The reactions were characterized as first order with respect to the protein and 11th order for the alcohols, irrespective of the kind of protein or alcohol. The rate constant was, however, significantly reduced by the branching of the alcohol structure and by the conversion of native ovalbumin to S-ovalbumin. The measurements were also performed on ovalbumin and S-ovalbumin whose SH groups were blocked from SH\ightleftharpoonsS–S exchange reactions by modification with iodoacetamides. By the conversion to S-ovalbumin, the modified protein also exhibited a considerable resistivity against denaturation, suggesting that SH\ightleftharpoonsS–S exchange reactions play no essential role in the formation of S-ovalbumin. On the basis of these results as well as determined activation parameters, the mechanism of the denaturation is discussed in the light of a combination of the hydrophobic interactions, between hydrophobic residues of the proteins and the alcohol molecules, and the fluctuations of the protein structures.