Spherical titanium suboxide nanoparticles (Ti5O9, Ti9O17, TiO) are synthesized by arc plasma. A mixture of Ar, H-2 and H2O at 10mbar pressure is used as the working gas. The variations of the proportion of H2O in the mixture gas during the synthesis induce a clear change in the size, color, composition, and optical properties of these nanoparticles. A reduction of H2O partial pressure reduces the grain size of the product. The titanium suboxide nanoparticles (Ti5O9, Ti9O17) show a blue luster. The UV-Vis-NIR absorption spectra present a substantial absorption in the range 400-800 nm. In particular, there is a uniform and stable absorption from 800 to 3000 nm. FT-IR spectra reveal the H2O partial pressure significantly affects the -OH groups content on the surface or in the bulk of the nanoparticles. (c) 2005 Elsevier B.V. All rights reserved.