Pulsed laser deposition (PLD) of MoO 3 thin films and their gas sensing properties are reported. As-deposited films were found to be a mixture of sub-oxides. Post-annealing of the films in air at 773K for 5h transforms the sub-oxides to MoO 3 with diminished intensities of (0 k 0) reflections. The microstructure of the films were investigated by scanning electron microscopy and atomic force microscopy (AFM). Sensors made of thin films (∼1μm) of MoO 3 deposited at 673K followed by post-annealing in air at 773K exhibit a high selectivity for ammonia in the temperature range of 508–555K with fast response (∼15s) and retrace times (∼300s)