Owing to the wide surface area, two-dimensional materials such as MoS2 are being used for chemical sensing of harmful gas molecules, which reduce the concentration of these pollutants to an environmentally acceptable value. Using density functional theory calculations, the adsorptions of SO2, SO3 and O-3 gas molecules on MoS2 monolayers were studied in terms of adsorption energy, charge transfer, band structures, and charge density differences. The results suggest that the adsorption of gas molecules on the MoS2 is energetically favorable, which gives rise to the most stable configurations. Besides, the gas molecules were physisorbed on the monolayer surface due to the weak interaction between them and MoS2. The O-O and S-O bonds of the adsorbed gas molecules were elongated after the adsorption process. This elongation of bond lengths is mostly attributed to the transfer of electronic density from the old bonds to the newly formed bonds between the molecules and MoS2. The band structure calculations indicate that the electronic properties of the MoS2 monolayers are altered upon the adsorption of gas molecules. Furthermore, the charge density difference calculations reveal the charge accumulation on the adsorbed molecules, which indicate the acceptor property of these gas molecules.