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Rutile-type complete solid solutions between TiO2 and SnO2 were directly formed as nanoparticles via the forced co-hydrolysis of precursor solutions of TiCl4 and SnCl4 under acidic hydrothermal conditions at 240°C. The change in the lattice parameters a0 and c0 of rutile (Ti, Sn)O2 solid solutions followed the Vegard's Law. The crystallite size of rutile-type TiO2 was varied from 118 to 22.5 nm by the formation of rutile-type solid solutions with 10 mol% SnO2. The crystallite size of the rutile-type solid solutions containing more than 20 mol% SnO2 was maintained in the range of 10–14 nm by the control of crystallite growth owing to the presence of SnO2. The specific surface area of the rutile-type solid solutions corresponded well to the change in the crystallite size. The optical band gap was gradually decreased with decreased SnO2 content in the rutile-type solid solutions. The beginning of the phase separation was observed in the sample Ti0.5Sn0.5O2 by heating at 1200°C above 1 h in air, since the rutile-type solid solutions were metastable in the large area of miscibility gap below Tc = 1430°C in the TiO2–SnO2 system.