Electrical conductivity and gas sensing properties of MoO₃ are investigated. The electrical conductivity is found to be independent of oxygen partial pressure in the temperature range 510–773 K. Two distinct conduction processes were identified from the conductivity experiments carried out under ambient air, moist oxygen, and moist argon. The conductivity in the low temperature range (510–578 K) are attributed to species arising from the reversibly inserted water molecules into MoO₃ lattice. The conduction process in the high temperature region (578–773 K) are attributed to the non-stoichiometry existing in the sample due to the presence of Mo⁵⁺ions which was confirmed by EPR and XPS investigations. Sensing characteristics of MoO₃ towards NH₃, H2, and LPG were studied. Experiments showed that the ammonia sensing mechanism of MoO₃ involved the formation of molybdenum suboxides and nitride.