Two-dimensional (2D) molybdenum disulphide (MoS2) atomic layers have a strong potential to be used as 2D electronic sensor components. However, intrinsic synthesis challenges have made this task difficult. In addition, the detection mechanisms for gas molecules are not fully understood. Here, we report a high-performance gas sensor constructed using atomic-layered MoS2ﾠsynthesised by chemical vapour deposition (CVD). A highly sensitive and selective gas sensor based on the CVD-synthesised MoS2was developed.ﾠIn situﾠphotoluminescence characterisation revealed the charge transfer mechanism between the gas molecules and MoS2, which was validated by theoretical calculations. First-principles density functional theory calculations indicated that NO2ﾠand NH3ﾠmolecules have negative adsorption energies (i.e., the adsorption processes are exothermic). Thus, NO2ﾠand NH3ﾠmolecules are likely to adsorb onto the surface of the MoS2. Theﾠin situﾠPL characterisation of the changes in the peaks corresponding to charged trions and neutral excitons via gas adsorption processes was used to elucidate the mechanisms of charge transfer between the MoS2ﾠand the gas molecules.