Molybdenum trioxide (MoO3) is of high sensitivity and fast response as a vital complement to H-2 sensors. To investigate the mechanism of gas detection ability of MoO3 layer, the adsorption of H-2, CO, NH3 and H2O gases molecules on MoO3 are studied by first principles calculations. Based on calculated adsorption energies, charge transfer and bonding mechanism, MoO3 shows a superior sensing performance to H-2 than CO, NH3 and H2O at room temperature, and new bonds with length of 0.9 A formed between H-2 and MoO3. However, in addition to H-2, MoO3 tends to detect NH3 at high temperature (750 K). A new O-H bond with length of 0.9 angstrom formed between NH3 and MoO3, and the charge transfer of -0.24 vertical bar e vertical bar is considerable greater than the result of -0.03 vertical bar e vertical bar at room temperature. An approach is proposed to calculate equivalent sensitivity of MoO3 by band gap difference, adsorption distance and charge transfer. The predicted sensitivities of MoO3 toward H-2, NH3, CO and H2O gases at varied temperatures are consistent with experimental results, demonstrating the validity and efficiency of our approach. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.