Strong nonideality observed in the osmotic pressure of concentrated globular proteins in aqueous media of moderate salt concentrations has long been associated with protein-protein interaction. As a consequence, virial expansions, based on the McMillan-Mayer theory, have been used to extract the physical phenomena observed. Our recent articles showed that a free-solvent model assuming hydration and salt binding are dominate factors for nonlinearity, successfully modeled BSA (67 kDa) and IgG (155 kDa) protein aqueous solutions at moderate salt concentrations. Similar findings for lysozyme (HEL, 14 kDa) and ovalbumin (45 kDa) are reported here. More significantly, the independently calculated hydration values regressed from the osmotic pressure data were compared with the solvent accessible surface areas of each protein investigated. The results showed that the hydration values determined from the free-solvent model are remarkably a monolayer equivalent of water.