Thermally grown molybdenum oxide films are amorphous, oxygen deficient and gas sensing. Air or vacuum annealing transforms them into a sub-stoichiometric MoO3−x phase. The samples annealed at 500 °C in oxygen were crystallized and identified as pure orthorhombic MoO3 phase. The conduction process and sensing mechanism of MoO3-x to ethanol have been studied.