Pyrolysis is usually modeled with nth-order or sigmoidal (nucleation or autocatalytic) kinetics and with parallel reactions having distributed activation energies: The underlying chemistry, however, consists of thermolytic bond cleavage, recombination reactions, and volatilization of low molecular weight (MW) products. A new approach is based on a dynamic population-balance equation for the molecular weight distribution of a pyrolyzing macromolecular material in a temperature-programmed thermogravimetric process. Random chain scission, repolymerization, chain-end scission yielding gas products, and loss of low-MW matter by vaporization are included in the model. Zeroth and first moments of the balance equation provide coupled differential equations for the time dependence of moles, n(t), and mass, m(t), of the sample. Separate changes in n and m account for the changes in the average molecular weight during pyrolysis. The activation energy for random chain scission is relatively larger than that for chain-end scission or repolymerization. The model simulates observations for temperature-programmed pyrolysis, including nth-order and acceleratory (nucleation) behavior.