AbstractThe degree of helicity θ of a series of homologous polypeptides as a function of solvent composition was investigated. The polypeptides studied were: poly‐N5‐(3‐hydroxypropyl)‐L‐glutamine (PHPG) as well as the corresponding 2‐hydroxyethyl and 4‐hydoxybutyl derivatives (PHKG and PHBG, respectively). PHPG, which is nonhelical in formic acid, attains helicity on addition of relatively small amounts of formates, formamide, and urea to its solution in formic acid. This demonstrates that the high acidity of pure formic acid is largely responsible for its helix‐breaking power‐probably through protonation of the peptide bonds. In formic acid‐water mixtures all three polymers show a maximum in degree of helicity at a mole fraction of about 0.3 formic acid. This is interpreted as being due to interaction between the two helix‐breaking solvents, which results in the formation of an inactive molecular species. It is shown that solvent‐induced transitions with helicity maxima are predicted by the Bixon‐Lifson treatment w