We model char formation in isothermal and thermogravimetric (TG) experiments by a catalytic one-step reaction. For isothermal experiments there is an analytic solution and we show how the maximum amount of char formed, and the stability of the char, varies with the parameters in our model. For TG experiments we use the concept of a ’characteristic temperature’ to provide bounds on parameter values and calculate the amount of char formed numerically. This approach can also be used to predict whether barrier-forming additives form char below or above the decomposition point of a polymeric solid.