Nanocrystalline Sn x Ti 1-x O 2 rutile solid solutions are important materials for a variety of gas sensors and catalysts. Although thermodynamic data are available for nanocrystalline SnO 2 and TiO 2 and for bulk (coarse-grained) Sn x Ti 1-x O 2 solid solutions, there is a lack of experimental thermochemical data on the energetics of Sn x Ti 1-x O 2 nanoparticles. In this work, Sn 0.586 Ti 0.414 O 2 rutile solid solution nanoparticles were synthesized. The surface energies of anhydrous and hydrated nanoparticles were measured by combining high-temperature oxide melt solution calorimetry molten 2PbO center dot B2O3 at 800 degrees C and water adsorption calorimetry. The surface energy of the anhydrous surface is 2.02 +/- 0.03 J center dot m-2, and that of the hydrated surface is 1.68 +/- 0.03 J center dot m-2. These values lie between the previously reported surface energies of rutile SnO2 and TiO2. The integral heat of water adsorption is -80 kJ center dot mol-1, with a chemisorbed maximum coverage of 6 H2O center dot nm-2. These values are also between those for TiO2 and SnO2 (rutile) reported previously. The strongly positive (unfavorable) energetics of mixing in SnO2TiO2 bulk solid solutions are predicted to change little at the nanoscale, and the extensive solid solution seen in the nanophase system prepared near room temperature reflects kinetic hindrance to exsolution of an initially homogeneous precipitate rather than thermodynamic stability.