Optimum band gap values, favourable band edge positions and stability in the electrolyte are critical parameters required for a semiconductor to have efficient photoelectrode properties. The present investigation carried out on the phase pure alpha & beta MoO3 thin film shows that the low bandgap beta-MoO3 possesses a mis-alignment with the water oxidation potential, while a more suitable band alignment is observed for the comparatively large bandgap alpha-MoO3. Both experimental and DFT calculations show that the valence edge of the orthorhombic (alpha-MoO3) phase is located at a higher energy (0.9 eV higher in VB-XPS and 1 eV higher in the DOS plots) than the monoclinic (beta-MoO3) phase, while the conduction edge value is roughly at the same energy level (-2.5 eV) in both polymorphs. Based on the above investigations, an all oxide heterojunction comprising of beta-MoO3/alpha-MoO3 is found to be suitable for improved PEC performance due to favourable energy band diagram and increased visible light absorption in beta-MoO3. Significantly higher cathodic photocurrent is observed for the beta-MoO3/alpha-MoO3 (1.6 mA/cm(2) at applied bias of -0.3V(RHE) under simulated 1 sun irradiation) as compared to the very low anodic response in beta-MoO3 (similar to 1.0 nA/cm(2)) and alpha-MoO3 (32 mu A/cm(2)). (C) 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.