The osmotic coefficients of 1,3-dimethylurea-h(2)(DMUh(2))/H2O and 1,3-dimethylurea-d(2)(DMUd(2))/D2O solutions (1, 2, 4, 12, and 20 m(aq), 15 < t/degrees C < 80) were obtained from differential vapor pressure measurements. Excess partial molar free energies, enthalpies, and entropies for the solvent and their isotope effects were calculated from the temperature derivatives of the osmotic coefficients. New partial molar volume data are reported at 25 degrees C at low and intermediate concentrations. The thermodynamic properties of solution are compared with those of urea and discussed using the cage model of hydrophobic hydration. The results support the mixed (polar-apolar) character of this compound and show that its structural effect on water changes with temperature and concentration.