A systematic comparison of chemical interactions and fire behaviour is presented for the thermoplastic elastomer (block copolymer styrene-ethylene-butadiene-styrene) (TPE-S)/diethyl- and methylvinyl siloxane (Si)/poly(phenylene oxide) (PPO), flame-retarded with aluminium diethylphosphinate (AlPi) and with ammonium polyphosphate (APP), respectively. TPE-S/APP/Si/PPO performed better in the cone calorimeter test (reduction in peak heat release rate from 2042 to 475 kW m(-2)), but TPE-S/AlPi/Si/PPO in the flammability tests (oxygen index (OI) and UL 94). This difference was caused by the different modes of action of APP (more in the condensed phase) and AlPi (mainly in the gas phase). Thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR) was used to analyse the mass loss and the evolved gas products, while a Linkam hot-stage cell to investigate the decomposition in the condensed phase. Moreover, a detailed analysis of the fire residues was done using solid-state NMR. C-13 MAS NMR showed that both flame-retarded compositions form graphite-like amorphous carbonaceous char, originating from PPO. P-31 MAS NMR and Si-29 MAS NMR delivered important information about interaction between phosphorus and the siloxane. For TPE-S/AlPi/Si/PPO aluminium phosphate and silicon dioxide occurred, while also silicophosphate was produced in TPE-S/APP/Si/PPO. The direct comparison of two of the most prominent halogen-free flame retardants containing phosphorus delivered meaningful insights into the modes of action and molecular mechanisms controlling flame retardancy. (C) 2016 Elsevier B.V. All rights reserved.