Ab initio calculations have been carried out for the PH2 + O-2 reaction by using Gaussian-2 theory, which is considered a key reaction to understand low-temperature phosphine combustion. It has been suggested that the reaction proceeds through two main routes at low temperatures. One is the reaction to yield OH radicals, and the other yields hydrogen atoms. Of the two reactions, the latter seems to be the chain branching process, which enables the spontaneous ignition of phosphine at room temperature.