Niobium (Nb) doped molybdenum trioxide (MoO3) thin films have been synthesized using spray pyrolysis deposition technique. The structural changes were observed with the help of X-ray diffraction technique. With increasing Nb concentration, the structure of MoO3 undergoes a phase transformation from alpha-orthorhombic to amorphous with nano-sized grains. The thread like reticulated morphology is converted into spongy like structure at higher Nb concentration (9 at% Nb). It is seen that Nb doping can lead to significant surface morphology changes in MoO3 films. It was found that the coloration efficiency increases with doping concentration. With increasing Nb concentration charge capacity, reversibility and electrochemical stability increases. The improvement is attributed to the amorphous structure of the doped samples that favors easy intercalation and deintercalation processes. Hence, we have successfully demonstrated formation of an adequate host for electrochromic devices with Nb (9 at%) doped MoO3 samples. (C) 2008 Elsevier B.V. All rights reserved.