The oxygen index (OI) of poly(butylene terephthalate) (PBT) tends to decrease when it is combined with milled-glass fibres either with or without the sizing treatment. This shows that the previously found apparent increase of flammability of PBT glass fibre composites (GFPBT) as compared to PBT is not due to the introduction of the flammable sizing together with the glass fibres in the polymer which was one suggested explanation in the literature, but rather to the wick and anti-dripping effects of glass fibres. The effectiveness of a typical brominated organic compound-antimony trioxide fire retardant system (FR), as measured by OI, is found to be larger in GFPBT as compared to PBT. A linear increase of the temperature index (TI) of PBT and of GFPBT is observed with increasing concentration of the FR. The fire retardant increases the time to ignite while it decreases the maximum rate of heat release and increases the smoke optical density and CO evolution on burning in the cone calorimeter. The dependence of fire risk and hazard assessement on the combustion model of the combustion test method is discussed for OI and cone calorimeter in the case of PBT, GFPBT and FR corresponding materials. © 1998 John Wiley & Sons, Ltd.