Uniformly dispersed ultrasmall tungsten oxide nanoparticles (W18O49) of sizes around 5-7 nm were synthesized using tungsten hexachloride as tungsten precursor and octadecylamine (ODA) as surfactant and as well as reducing agent. The as-synthesized nanoparticles (NPs) were characterized thoroughly by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and N-2 adsorption desorption isotherm (BET). From the XRD patterns, formation of monoclinic primitive phase of W18O49 was observed while TEM images showed well dispersed particles of sizes 5 7 nm. The surface area of the W(18)O(49)9 NPs was found to be 27.17 m(2) g(-1). These ultrasmall W18O49 NPs have been studied as photocatalysts for the first time in the degradation of methylene blue (MB). The photocatalytic activity was evaluated in oxidative degradation of MB with H2O2 under solar irradiation. The particles exhibited pronounced activity in degradation of MB as well as efficient recyclability. The small band gap energy of W18O49 NPs and their large surface area helps in the production of higher electron (e(-)) and hole (h(+)) pairs which in a way also prevents the e(-) and h(+) pairs from recombination within the nanoparticles. This greatly improves and enhances the photocatalytic activity of our synthesized nanoparticles. (C) 2016 Elsevier B.V. All rights reserved.