We used pressure perturbation calorimetry (PPC), a relatively new and efficient technique, to study the solvation and volumetric properties of amino acids and peptides as well as of proteins in their native and unfolded state. In PPC, the coefficient of thermal expansion of the partial volume of the protein is deduced from the heat consumed or produced after small isothermal pressure jumps, which strongly depends on the interaction of the protein with the solvent or cosolvent at the protein-solvent interface. Furthermore, the effects of various chaotropic and kosmotropic cosolvents on the volume and expansivity changes of proteins were measured over a wide concentration range with high precision. Depending on the type of cosolvent and its concentration, specific differences were found for the solvation properties and unfolding behaviour of the proteins, and the volume change upon unfolding may even change sign. To yield a molecular interpretation of the different terms contributing to the partial protein volume and its temperature dependence, and hence a better understanding of the PPC data, molecular dynamics computer simulations on SNase were also carried out and compared with the experimental data. The PPC studies introduced aim to obtain more insight into the basic thermodynamic properties of protein solvation and volume effects accompanying structural transformations of proteins in various cosolvents on one hand, as these form the basis for understanding their physiological functions and their use in drug designing and formulations, but also to initiate further valuable applications in studies of other biomolecular and chemical systems.