The thermal decomposition of four commercial rigid polyuretnane foams(two of which contain organophosphorus flame retardants) has been studied over the temperature range 200 to 1000C using elemental analysis and gas chromatography. The general decomposition mechanisms of the foams appear to be similar and involve a preferential release of some of the polyol content followed by a uniform fragmentation of the residue to release particulate material. This particulate material (smoke) appears to volatilise from the furnace zone at temperatures up to 600C but decomposes above 700C to give the typical family of nitrogen containing products of low molecular weight (hydrogen cyanide, acetonitrile, acrylonitrile, pyridine and benzonitrile)as observed with flexible polyurethane foams. The main nitrogen containing product of low molecular weight observed in this work is hydrogen cyanide which increases markedly in yield from 700C to the maximum temperature studied (1000C). At 1000C the yields of hydrogen cyanide from the foams range between 3.8 to 7.3 weight per cent conversion which represents 28 to 42 per cent of the theoretically available nitrogen.