Pyrolysis of two representative poly(aryl ether sulfones) (PAES) with anti without the isopropylidene link revealed its important role in controlling the overall thermal stability. This was probed quantitatively by the development of mechanistic pyrolysis models for single components and model compound mixtures of phenyl sulfone (PS), phenyl ether (PE) and 2,2-diphenylpropane (DPP). Model development on the computer allowed "on-the-fly" calculation of species’ properties using computational quantum chemistry. This also provided quantitative values of model parameters. The reactivity differences of the polymers were interpreted in terms of the predicted changes in the product spectra when PS and PE were pyrolyzed with and without the reactant DPP. The increased selectivity to benzene and the decreased selectivity to (phenylsulfonyl)biphenyl with DPP present suggested an increase in the ratio of bond scission to bond formation that accounted for the decreased tendency of the isopropylidene-containing PAES for molecular weight increase and gel formation, two global measures of thermal stability.