Man encounters increased hydrostatic pressures during diving and while under hyperbaric therapy. The effects of pressure on organisms contribute to their zonation along the length of the water column of the oceans. Changes in hydrostatic pressure affect processes such as microtubule formation, ribosome aggregation of protein synthesis, and cell division to an extent easily observed in the laboratory. Theromodynamic explanations and predictions of pressure effects can be made if values for partial model volumes and compressibilities and of biologically important molecules were known or could be predicted. It is the purpose of the proposed research to determine and to study values of partial molal volumes and compressibilities at high pressures. The partial molal volume will be calculated from the density of solutions measured at pressures to 2,000 atm. The methods used will be similar to that described by Yayanos (1972) at high pressures. The density of solutions at 1 atm will be measured as a function of temperature using dilatometers and a commercially available densimeter. These density measurements yield values on the PVT (pressure-volume-temperature) surface of the solutions. The PVT values will be fitted to at least 12 equations of state to determine which one reproduces the data most accurately. The observed values of partial molal volumes will also be studied in the light of theories of solvation, dipole-dipole interactions; micelle formation, ion pair formation and water structure.