Hydroquinone bis(di-2-methylphenyl phosphate) (HMP) is an effective phosphorus flame retardant, but its gas flame retardant mechanism is not clear. In this study, the thermal degradation of HMP was investigated by thermogravimetry (TG) coupled with Fourier transform infrared spectroscopy (FTIR) under nitrogen and air, respectively. The results of the FTIR and semi-quantitative analyses agreed with TGA and derivative thermogravimetric analysis. Under nitrogen, the results of the TG, FTIR, and semi-quantitative analyses showed that HMP decomposed into benzyl alcohol and hydroquinone phosphate ester in a one-step process from 392 to 475 A degrees C. Under air, the TG, FTIR, and semi-quantitative analyses showed that HMP decomposed in a two-step process. In the first step from 385 to 452 A degrees C, HMP decomposed into benzyl alcohol, hydroquinone phosphate ester, carbon dioxide, water, and alkyne, while in the second step from 491 to 800 A degrees C, it decomposed into carbon dioxide, water, and alkyne.