The thermal degradation of aluminium diethylphosphinate (AlPi) with aluminium trihydrate (ATH), two flame retardant additives, was investigated. The interactions between the additives were characterized using thermal analysis (TG). The evolved gas were analysed by TGA-FTIR and the degradation products formed in the condensed phase were fully characterized using solid state NMR analysis, and in particular 2D NMR, as well as XRD analyses. Decomposition pathways have been determined and it was proposed that chemisorption of the phosphinate on alumina resulting from the ATH dehydration, activated by the temperature increase, influences the degradation of the mixture. As a consequence, the degradation products of AlPi differ in presence of ATH since the formation of aluminophosphonate is prevented. (C) 2012 Elsevier B.V. All rights reserved.