Blue-colored aqueous colloids of n-doped tin dioxide nanoparticles in the 4-9-nm size regime have been prepared hydrothermally either by inducing oxygen vacancies into the SnO2 lattice or by doping the nanoparticles with antimony. Autoclavation at temperatures above 250 degrees C is found to be necessary to achieve n-doping of the particles. Blue-colored oxygen-deficient nanoparticles are obtained in the absence of antimony by employing a reducing atmosphere inside the autoclave..If these colloids are exposed to air, their blue color vanishes within 1 day, indicating back reaction of the vacancies with oxygen. Antimony-doped tin dioxide nanoparticles have been prepared by using either (SbCl3)-Cl-III or (SbCl5)-Cl-V as the source of antimony. In contrast to the oxygen-deficient tin dioxide, the blue color of antimony-doped nanoparticles is stable in air. The blue color of the colloids corresponds to a broad absorption peak in the red and IR regions, independent of the method of n-doping. As for bulk material, this IR absorption is interpreted as a plasma excitation of free carriers in the n-doped nanoparticles. High-resolution TEM images and X-ray powder diffraction patterns confirm the high crystallinity of the nanoparticles and the same rutile lattice structure as known from bulk SnO2.