Despite the SnO2-TiO2 system being a classical example of immiscibility and spinodal decomposition for over 50 years, the enthalpies of mixing in this rutile-structured solid solution have not been measured previously because of the refractory nature of the oxides. We report new measurements of enthalpies of drop solution in molten 2PbO center dot B2O3 at 800 degrees C of the SnxTi1-xO2 solid solutions system. The data show a nearly symmetrical deviation from linearity, supporting regular solution behavior with an interaction parameter of 27.0 +/- 2.6 kJ/mol. The calculated miscibility gap has a critical temperature of 1351 degrees +/- 156 degrees C. These calorimetric data are in reasonable agreement with those calculated from phase equilibria and suggest that the regular solution model, which implies random mixing of the cations and no excess (vibrational) entropy of mixing, indeed adequately describes the thermodynamic behavior. The interaction parameter is more positive than that for oxide systems of lower cation charge and similar volume mismatch.