We describe a mathematical model to evaluate the retardancy effect of inert additives on the flammability parameters of thermally thin thermoplastics in radiative ignition experiments. The concept of a critical mass flux from the solid phase into the flame is used as the criticality condition for defining ignition. Kinetic parameter values are chosen by relating the Arrhenius parameters to the 'characteristic temperature' measured in thermogravimetric experiments (TG). The specific application that we have in mind for our model is piloted ignition in the Cone Calorimeter. © 1996 Elsevier Science Limited.