We investigated the pressure effect on the helix‐coil transition of Ala‐rich peptides in aqueous solutions by FT‐IR spectroscopy. The population of the helix conformer increased with increasing pressure. The differences in the partial molar volume between the &agr;‐helix and random coil conformers of Ala‐rich peptides are determined from the pressure dependence of the absorbance. The partial molar volume of the helix conformer is smaller than that of the coil conformer. The present result is different from the previous study (H. Noguchi,Prog. Polym. Sci. Jpn., 1975, 8, 191–199.), which reported that the partial molar volume of the helix form of polypeptide is larger than that of the coil form. © 2004 American Institute of Physics