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Thermoelectric properties of undoped and doped (Ti0.75Sn0.25)O2 were investigated for high-temperature thermoelectric conversion application. Nano-composites were formed by annealing above 1000°C. Outside the spinodal dome, ilmenite-type SnTiO3 precipitated from the rutile structure. Thermoelectric properties were measured in the temperature range from room temperature to 1000°C. (Ti0.75Sn0.25)O2 was doped with both acceptor and donor dopants. Both undoped and doped (Ti0.75Sn0.25)O2 exhibit n-type electrical behavior independent of the type of the dopant. The electrical conductivity was enhanced three orders of magnitude by donor doping with Nb2O5 or Ta2O5; achieving a maximum of 546 S/m at 850°C. The increase in electrical conductivity was accompanied by reduction of the absolute Seebeck coefficient. Seebeck coefficient reduction of −600 μV/K was observed between undoped and 4% Ta2O5 doped samples. The solid solution and doping reduced the thermal conductivity to <4 W/mK, far below the parent materials TiO2 and SnO2. Lattice thermal conductivity decreased with increasing temperature, achieving 1.9 W/mK at 900°C for 4% Ta2O5 doping. No further reduction in thermal conductivity was observed in annealed samples containing nano-sized SnTiO3 precipitates. Dimensionless figure of merit (ZT) attained was <0.1.