Molecular conformational equilibrium is the most central concept in the chemistry and biochemistry of chain molecules. This equilibrium is occasionally affected by environmental conditions such as temperature, pressure and solvents. Such structural flexibility of molecules plays an important role in chemical and biological phenomena in liquid phase. In particular, the equilibrium in water is of vital importance for biological systems.The pressure effect on the equilibria in liquids can give information of volume differences between trans/gauche of rotational isomers or native/unfolded states of proteins. The volume properties are important to understand the intermolecular interaction between the solute and solvent molecules and the molecular mechanism. It is accepted that the volume changes for trans/gauche equilibrium of rotational isomers in non‐polar solvents are less than −5 cm3/mol and for native /unfolded states of proteins in aqueous media less than −100 cm3/mol.Recent development of high pressure FTIR spectroscopy combined with resolution enhancement techniques and Raman spectroscopy is able to detect the signal of each rotational isomer in dilute aqueous solution and the secondary structure of the pressure induced structure changes of proteins in water. In this study, the effect of pressure on the conformational equilibrium between rotational isomers of haloacetone, alanine dipeptide, and proteins in aqueous media has been studied by the FTIR and Raman spectroscopes. On the base of both observed volume changes of simple chain molecules and proteins, the molecular mechanism on the pressure induced conformational changes will be discussed. © 2004 American Institute of Physics