2-Methyl-1,2-oxaphospholan-5-one 2-oxide (OP) is useful in a wide range of flame-retarded compositions. It is a thermally stable substance which, in inert atmosphere, looses weight at 200degreesC on dynamic heating, leaving a solid residue, the amount of which is sensitive to the presence of moisture. Due to the reactive P-O bond, the thermal stability of OP can be increased by reaction with amines or alcohols and it can be introduced into a variety of plastics. Upon pyrolysis at 450degreesC, OP volatilizes and decomposes, producing CO2, low-chain hydrogencarbons, aliphatic and aliphatic-aromatic phosphine oxides and a black deposit, where polymeric phosphine oxides and phosphorus acid anhydrides were detected. Mechanism to explain the occurrence of the different products is proposed. Based on these data the condensed- and vapor-phase fire retardant mode of action of OP appear to be feasible. (C) 2003 Elsevier B.V. All rights reserved.