17O NMR spectroscopy and 13C NMR spectroscopy have been used to study the mechanism of interaction of sugars with bovine and caprine caseins in D2O. The 17O NMR relaxation results showed in all cases an increase in water of hydration, as a result of added sugar; this was predominantly associated with 'trapped' water in the caseins. Analysis of the virial coefficients, obtained from the 17O relaxation data, suggested that preferential interactions occur in the sugar - protein solutions. This could be the result of either sugar binding or a solute - solute thermodynamic effect, preferential hydration. The addition of sugars to deuterated solutions of bovine casein and caprine casein high in α(s1)-casein had little or no effect on either line width or chemical shilfts of the 13C NMR spectra of these milk proteins. 13C NMR studies of sucrose, at various concentrations (100-300 mM) in the presence of caprine casein high in α(s1)-casein, showed no changes in either chemical shifts or T1 values. This indicates that the sugar molecules tumble isotropically and therefore neither bind to the protein nor affect viscosity in the protein - sugar studies. All of these data suggest that the preferential exclusion of the sugar from the domain of the caseins results in preferential hydration of the caseins.