Partial pressures of C-60 and C-70 were measured as a function of temperature at several compositions from pure C-60 to pure C-70 in the C-60-C-70 binary system. The measurements were carried out in the temperature range 600-800 K by using Knudsen effusion mass spectrometry. The pressure-temperature-composition data suggest that the two fullerenes are soluble in each other in the solid state to the extent of about 30%, with a miscibility gap in between. A binary phase diagram is reported on the basis of these studies. The partial pressures of both C-60 and C-70 in equilibrium with fullerite were also measured as a function of temperature. The enthalpy and entropy of vaporization of each species are derived from the pressure-temperature data for every solid phase studied. The thermodynamic data suggest that C-60 and C-70 are more strongly bound in fullerite than in the solid solution or in the pure phases. This indicates the influence of higher fullerenes.