The dependence of the thermal stability of high-performance poly(arylether sulfones) (PAES) on the initial molecular weight distribution and backbone structure was assessed experimentally and through computer simulation. Reaction of PAES polymers resulted in the formation of an insoluble gel fraction and significant changes in weight and number average molecular weights of the sol fraction. A PAES with alternating ether and sulfone linkages formed a larger fraction of gel at a given reaction time than a PAES with the hydroquinone moiety. For a given chemical composition, more rapid molecular weight changes and gel fraction formation were observed for the polymer with the higher value of the initial weight average molecular weight. The growth of molecular weight was also faster for the polymer with the broader initial distribution. The simultaneous increase in M(w) and decrease in M(n) suggested the occurrence of two types of overall reactions : scission and addition. Simulation of these reactions using Monte Carlo kinetics allowed estimation of the range of probability for bond scission, R, of 0.5 < R ( 0.8 capable of accounting for the observed experimental behavior. The dependence of the simulated molecular weight changes on the initial molecular weight distribution agreed qualitatively with the experimental trends.