We have investigated the effect of pressure on the conformational equilibria of 1-chloropropane and 1-bromopropane in water and organic solvents using Raman spectroscopy. In particular, we focus on 1-chloropropane and 1-bromopropane in water as a model for characterizing the hydrophobic effect on the molecular conformation. From the pressure dependence of the Raman intensities of the carbon-halogen stretching bands, the volume differences (Delta V-t -> g) between trans and gauche conformers of 1-chloropropane and 1-bromopropane in water and organic solvents are determined. All the values are found to be negative. The values of Delta V for aqueous solutions deviate significantly from the expected results obtained for organic solvents. The Delta V is divided into several contributions and interpretation of these contributions indicates that the significantly large negative Delta V for aqueous solutions is explained by the considerably low packing density of solvent water compared to the density of the organic solvents. The results for the aqueous solutions are compared with the pressure effect on the dimerization of methane in water calculated by Hummer et al. (Hummer, G.; et al. Proc. Nad. Acad. Sci. U.S.A. 1998, 95, 1552.). Our results strongly support the validity of the increasing desolvation barrier with increasing pressure predicted by Hummer et al. Finally, we discuss the biological relevance of the results for aqueous solutions in terms of the pressure denaturation of proteins.