AbstractPartial compressibilities of globular proteins in water are reviewed. Contribution of hydrational and of intrinsic compressibilities to experimental partial quantity have been evaluated from ultrasonic data using two independent methods: (a) additive calculation of the hydrational contributions of the surface atomic groups and (b) an analysis of correlation between partial compressibility and molecular surface area. The value (14 ± 3) × 10−6bar−1for the isothermal compressibility coefficient of the protein interior at 25°C was obtained as an average value for variety of globular proteins. This value is similar to that of solid organic polymers. Possible relaxation contribution to partial compressibility is roughly estimated from comparison of thermodynamic with x‐ray data on protein compressibility. The average compressibility of water in the hydration shell of proteins was found to be 35 × 10−6bar−1, which is 20% less than that of pure water. © 1993 John W