Thin films of molybdenum oxide have been deposited by reactive magnetron sputtering and characterised by photoelectron spectroscopy. Such films of MoOx are increasingly being used as electrode interfaces in both organic light emitting diodes and bulk heterojunction polymer-fullerene solar cells. Control of the Mo to oxygen stoichiometry has been gained by two methods of R.F magnetron sputtering. The first method is by controlling the proportion of oxygen in the sputtering chamber and the second is by controlling the level of surface oxidation of the sputter target prior to deposition, both methods have been used to control average film stoichiometry. In general the transition from metallic like film to oxide like film occurs at an oxygen chamber partial pressure of 15 %. UPS measurements show that such a transition, and resulting increase in the average oxidation state, leads to a shift in the Fermi level from 4.5eV to 5.2eV with respect to vacuum. The presence of specific oxidation states below +6 give rise to electronic structures that lie between the valence band edge and the Fermi level. For reduced samples +2 and +5 oxidation states peaks appear at binding energies of 0.4eV and 2.1eV respectively while a third peak at 1eV could be attributed to either the presence of +3 or +4 oxidation states.