The excess volumes of the binary system of ribonuclease A (RNase A) with water were obtained as a function of composition at 25 degrees C. The excess quantities for RNase A were compared with the published data for several unrelated proteins (lysozyme, serum albumin, lactoglobulin, and chymotrypsinogen A). The hydrophobicity of these proteins is gradually changed over a wide range. It was found that the more hydrophilic a protein is, the more significant the hydrophilic hydration contribution is. RNase A is the most hydrophilic protein in the present study, and it has the most significant hydrophilic hydration contribution. (C) 2014 Elsevier B.V. All rights reserved.