S-terminated MXenes have been recently successfully synthesized in experiment (Science, 2020, 369, 979). In particular, the atomic arrangement of M₂CS₂ (M = Sc or Y) is different from all the already known MXenes. Motivated by this, we have theoretically studied the adsorption of 17 kinds of gases (including atmospheric and organic molecules) on M₂CS₂. Our results revealed that the adsorption of N/O-containing gases on M₂CS₂ are preferable, and the Y₂CS₂ exhibits the high chemical activity as compared with Sc₂CS₂. In detail, the Sc₂CS₂ could only effectively capture the NH₃ and nicotine, while other gases (including NO_x, ethanol, acetone and methanol) could also be captured by Y₂CS₂ because of their strong interaction strengths (E_ads < −0.50 eV). In addition, by considering the adsorption energies, charge transfers, density of states and current/voltage relationships of M₂CS₂-gas systems, we found that Sc₂CS₂ could be the potential candidate as NH₃/NO/nicotine/ethanol gas sensor, and the Y₂CS₂ exhibits the extremely high sensitivity and selectivity towards NO. Importantly, the M₂CS₂ is reusable after gas sensing process because of the short recovery times (<0.04 s) at room temperature. Our results will be helpful in understanding the chemical properties and the potential applications of such unique MXene phase (M₂CS₂).