As is well known, gas phase flame retardant mechanisms are very hard to be understood for the extremely intricate burning process but they are crucial for developing highly efficient flame retardants. Pyrolysis provides a valid way for this purpose because combustion proceeds after it. In this work, the pyrolysis behaviors of aluminum hypophosphite (AHP), aluminum diethylphosphinate (AIPi), poly(1,4-butylene terephthalate) (PBT) and PBT composites (with 10 wt% AHP or AIPi) were investigated by the fragment-free pyrolysis vacuum ultraviolet photoionization time of flight mass spectrometry (PY-PI-TOFMS) at different temperatures in real time. The gas phase flame retardant mechanism of AHP in PBT is proposed to be the flame inhibition effect of PH3, H3PO2 and P-4 pyrolyzed from AHP. That of AIPi in PBT is speculated to be phosphorus-containing compounds and a kind of phosphorus radical pyrolyzed from AIPi as flame inhibitors and the promoted pyrolysis of AIPi by PBT matrix to be synchronous with it. The eight-membered cyclic non-covalent adduct of C2H5-(O=)P center dot-OH radical and a molecule of diethylphosphinic acid via intermolecular double hydrogen-bond interaction can produce PO type radicals more effectively in flame. Thermogravimetric analysis results are in accordance with those of pyrolysis. And the much better gas phase flame retardant effect of AIPi than AHP in PBT concluded from combustion tests are interpreted by pyrolysis results, which provides powerful guidance for developing highly efficient flame retardants. (C) 2019 Published by Elsevier Inc. on behalf of The Combustion Institute.