An approach has been suggested to study the H/D isotope effect on protein-water and protein-protein intermolecular interactions by determining the content of non-freezing water using low-temperature (1)H NMR in mixed (H2O/D2O) water solutions. Direct data are obtained on the amount of H2O adsorbed (absolute hydration) in presence of the heavy isotope (deuterium D), and isothermals of H2O/D2O fractionation at protein surface groups are presented for temperatures between -10 degrees C and -35 degrees C and solutions of varying composition. The fractionation factor, phi = [x/(1 - x)]/[x(0)/(1 - x(0))], where x and x(0) are the fractions of deuterons in hydration and bulk water, respectively, appeared to be extremely high: phi >> 1 at 0.03 < x(0) < 0.10. The high values of phi indicate a decrease in apparent hydration of protein molecules. A probable reason of the effect can be an inter-protein molecular solvent-mediated interaction induced by D2O. The excess of phi over 1 appears to provide a quantitative estimate of the fraction of hydration water affected by such interaction.