Calorimetric techniques are important in studies of interactions between solutes and solvents. Our laboratory has for many years been involved in the measurement of enthalpies of solution in water, at different temperatures, of simple organic compounds. Results of such work are of a particular interest in discussions of thermodynamic properties of biochemical systems. During recent years the work has concentrated on studies of the hydrophobic hydration and microcalorimeters for dissolution of slightly soluble gases, liquids and solids have been developed. The different solution microcalorimeters have now been incorporated into a modular system where the different vessels all are modifications of a microtitration vessel which was reported earlier. Much of this development work has been based on joint projects with S.J. Gill, Uni- versity of Colorado, Boulder, Colorado. A~~~~H~ = A~~~H~ - A H@ VaP where A H@ is the enthalpy of vaporization to form the ideal gas. Enthalpies of vaporization are, preferably, obtained from calorimetric measurements or they can be calculated from accurate values for the temperature variation of the vapor pressure. Enthalpy determinations at constant pressure but at different temperatures will lead to values for corresponding changes in heat capacities. VaP