With a Tian–Calvet microcalorimeter, heats of dilution at 25 °C have been measured for aqueous and acid (0.2 NHCl) solutions of polyacrylic acid and of propionic acid, and for aqueous solutions of the sodium salts of polyacrylic acid. The results are interpreted in terms of the change in structure of water resulting from the lengthening of the solute molecule. The heat of dilution of moderately concentrated solutions of sodium polyacrylate can be explained by the theory of Eigen and Wicke.