The aim of this study was to investigate the thermal degradability, and in particular, the thermal degradation mechanism of organophosphorus flame-retardant poly(methyl methacrylate) (PMMA). For this purpose thermogravimetry and direct pyrolysis mass spectrometry analyses were used. Release of diethylphosphinic acid, melamine, and several products involving Al-O-P and N-P linkages were detected from the organophosphorus additive containing aluminium diethylphosphinate, melamine polyphosphate and zinc borate. When incorporated in PMMA, reactions of diethylphosphinic acid, melamine and/or their derivatives with the ester group affected the decomposition pathways by generation of (C(2)H(5))(2)POOCH(3) and HNCO at relatively high temperatures. (C) 2011 Published by Elsevier Ltd.