In order to understand the effect of flame retardant (FR) and blowing agents on the thermal stability of rigid polyurethane foams and their resultant chars, two series of polyurethane foams produced with different blowing agents (HCFC-141b and pentane) and various concentrations of a FR (0-50 wt%) were investigated using, standard flammability test (ASTM, D-3014), solid-state C-13 NMR, TGA and Py-GC/MS. The unique combination of these analytical techniques has proved to be a valuable method for understanding the thermal degradation of rigid polyurethane foams. The standard flammability tests indicate an optimum FR concentration of about 15 wt% for foams using HCFC-141b as the blowing agent, while no optimum condition was determined with pentane. The percent mass retained (PMR) values or char yields have a linear relationship with combustion flame temperature in both series of blowing agents. The solid-state C-13 NMR studies clearly show that pentane is chemisorbed during the polymerization and is retained within the foam matrix. The chars have lower concentrations of methylene and oxygenated aliphatic carbons, but a subsequent increase in aromatics is observed. The FR investigated preserves the chemical structure of the polyurethane foam, and, therefore, results in a higher PMR or char yield. The TGA experimental data showed that the maximum combustion reactivities of the chars have a linear relationship with the FR concentration in the parent foams. Py-GC/MS results indicate that the aliphatic oxygenated functional groups are the first to evolve during the pyrolysis and combustion of the polymeric structure. Finally, this study has shown that the addition of FR to the foam formulation results in lower concentrations of small molecules being volatilized, and therefore, preserving the original chemical structure of the parent foam. However, the FR investigated does not seem to be as effective for the pentane series, and gives higher char aromaticities and PMR values than those reported for the HCFC-141b series. (C) 2002 Published by Elsevier Science Ltd.