Nanocrystallites of antimony-doped tin dioxide have been prepared hydrothermally by treating colloids of tin antimony oxide in an autoclave. At a synthesis temperature of 270 degrees C stable colloidal solutions of blue-colored SnO2:Sb nanocrystallites have been obtained. High resolution transmission electron microscopy (TEM) images show highly crystalline particles in the 4 to 9 nm size regime. X-ray powder diffraction patterns of the nanocrystals indicate the same rutile lattice structure as known from bulk SnO2. Powders of the antimony-doped nanocrystals exhibit an up to 10(5) fold increase in electrical conductivity as compared to the corresponding undoped systems. The blue color of the doped colloids corresponds to a broad absorption peak in the red and the IR region. The IR-absorption spectrum of doped nanoparticles deposited onto sapphire substrates has been fit to a model based on the Drude theory of a free electron gas in SnO2:Sb and an effective medium approximation. The model indicates that the IR absorption corresponds to a plasmon polariton excitation of weakly interacting n-doped nanoparticles.