Conducting antimony-doped tin oxide (ATO) nanoparticles are prepared by a nonaqueous solution route, using benzyl alcohol as both the oxygen source and the solvent, and tin tetrachloride and various Sb(III) and Sb(V) compounds as tin and antimony sources, respectively. This reaction produces nonagglomerated crystalline particles 3−4 nm in size, which can be easily redispersed in high concentrations in a variety of solvents to form stable transparent colloidal solutions without any stabilizing agents. The synthesis temperature is the most important processing parameter largely governing the reaction course and the particle properties, while the nature of the antimony source has only a marginal influence. The cassiterite SnO2 lattice can accommodate up to 30 mol % antimony without significant changes in the structure. The incorporation of an increasing percentage of antimony causes a continuous decrease in particle size and a slight asymmetric lattice distortion. The introduction of an antimony dopant dramatically increases the particle conductivity, which reaches a maximum for 4% antimony, being more than 2 orders of magnitude higher than that of the pristine SnO2 nanoparticles. The obtained conductivity of 1 × 10−4 S/cm is the highest ever reported for the nonannealed nanosized ATO particles. Annealing in air at 500 °C further improves the conductivity to 2 × 102 S/cm, because of the particle sintering. Exceptionally high conductivity, small size, narrow size distribution, and dispersibility in various organic solvents make the ATO nanoparticles excellent primary building units for assembling nanostructured transparent conducting oxide materials with defined porous architectures.