The retardancy effect of additives, undergoing an endothermic decomposition in the solid phase, upon flammability properties of thermally thin thermoplastics is investigated. The criterion for ignition is that of a critical mass flux of volatiles pyrolysed from the solid into the gas phase. Strong coupling between the solid and gas phases is thus neglected in this model. The kinetic parameter values are set up in such a way that easy comparison can be made between the cone calorimeter and thermogravimetric analysis (TGA). The requirements for an additive to have maximal effect on ignition time and maximal mass loss rate are found to contrast, so that a simple heat-sink additive cannot maximise both. © 1997 Elsevier Science Ltd.