Titanium dioxide is a wide band gap (3.2 eV) semiconductor which is photo-active when irradiated with UV light. For wider scale use of TiO2 as a photocatalyst, its activity needs to be extended to the visible light region (constituting 45% of total incident solar energy). A diffusion flame aerosol reactor (FLAR) with an oxygen lean environment in the particle formation zone has been used to synthesize oxygen deficient titanium suboxide (TiOx with x < 2) nanoparticles. Using a standard-based electron energy loss spectroscopy (EELS) technique, the non-stoichiometry (x in TiOx) in the flame synthesized particles has been quantified with high accuracy (uncertainty less than 3%). Under an oxygen lean environment in the particle formation zone, the non-stoichiometry in the TiOx particles is a function of the flame temperature. The value of x in the flame synthesized TiOx nanoparticles is in the range of 1.88 < x < 1.94. Diffuse reflectance spectra confirmed that the oxygen deficient TiOx particles absorbed visible light. Visible light activity of the TiOx particles is demonstrated by photocatalytic degradation of methyl orange solution under visible light illumination. (C) 2008 Elsevier B.V. All rights reserved.