Polymer materials with different surface-to-volume ratios require different mechanisms of flame retardants regarding condensed phase and gas phase activity. The flame retardant formulations in poly(ethylene terephthalate) (PET) are investigated regarding a condensed phase and gas phase activity by using thermogravimetric analysis (TGA), TG-mass spectrometry (MS), TG-Fourier transform infrared (FTIR), UL94, cone calorimeter and scanning electron microscopy–energy-dispersive X-ray spectrometer measurements. The flame retardant formulations containing phosphates, phosphonates, and phosphinates as flame retardants are analyzed by using a simultaneous analysis consisting of a differential thermal analysis-TGA device which is in situ coupled to FTIR and MS. All analysis methods show a gas phase activity for the phosphonate (PCO 910), a condensed phase activity for the phosphate (3,9-bis(phenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro-5,5-undecane-3,9-dioxide, (SPDPP) and a mixed condensed and gas phase activity for the new synthesized phosphate and 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide containing flame retardant 3,9-bis(phenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro-5,5-undecane-3,9-dioxide (SPDPDOM). The fire behavior of PCO 910 can be improved by adding O,O'-Terephthaloyl-bis-N,N'-naphthalimide ester as NOR radical-forming agent (NOR-RF) reaching a total amount of 3 wt % of both active agents for a UL94 V-0 classification in PET
