Group-IV phosphide monolayers (MP, M=C, Si, Ge and Sn) provide a versatile platform for photocatalysts, as well as optoelectronic and nanoelectronic devices. Herein, comprehen- sive first-principles calculations and ab initio molecular dynam-ics (AIMD) simulations were performed to explore their stabilities in the air. We identified that the MP monolayers have excellent mechanical properties and their carrier mobilities are higher than that of phosphorene. The MP monolayers were predicted to possess superior oxidation resistance than the boron phosphide (BP) monolayer based on the proposed donation–backdonation theory. It was observed that the dissociation and chemisorption of a water molecule on the monolayers are kinetically difficult both in the water and in oxygen–water environments involving energy barriers of 1.28–3.48 eV. We also performed AIMD simulations at 300, 1000,1200 and 1500 K. It is noteworthy that only the carbon phosphide (CP) monolayer can retain an intact structure at 1500 K, while the other three monolayers can just sustain to 1200 K. These results provide a guidance for their practical application and experimental fabrication.