The effect of water-miscible halogenoalcohols, alkylalcohols, alkoxyalcohols and aminoalcohols on the conformational stability of bovine serum albumin (BSA) and egg white lysozyme (Lys) has been examined by o.r.d. to find out why 2-chloroethanol binds preferentially to protein at low concentrations and acts as a strong helix-forming reagent. The results have been summarized as follows; (1) at low alcohol concentrations the order of helix-forming ability for BSA is halogenoalcohol > alkylalcohol > other substituted alcohols; (2) on Lys, for all concentrations, the halogenoalcohols are stronger helix-forming reagents than the others. In order to see if hydrogen bonding of the halogenoalcohols might be responsible for the denaturation, the molecular interaction between the above alcohols and N-acetylglycine-N,N′-dimethylamide, N-methylacetamide and N,N-dimethylacetamide in solutions of carbon tetrachloride were studied by i.r. and 1H n.m.r. spectroscopy.Our results show that only the halogenoalcohols have a proton-donating ability strong enough to break the inter-amide hydrogen bonds and form hydrogen bonds with the amide carbonyl, and 2-halogenated ethanols may act as bifunctional hydrogen-bonding solvents. Thus it has been concluded that, in the denaturation of protein by halogenoalcohols, the formation of hydrogen bonds with the protein should be considered as a predominant factor inducing conformational changes in the protein.