Poly(m-phenylenediamineisophthal)amide (MPD-I) and poly(p-phenylenediamineterephthal)amide (PPD-T) are among the most important high-tech polymers. The degradation of three MPD-I fibers with different molecular weight distributions and a PPD-T fiber was studied in a proprietary laboratory degradation chamber. The results were fitted satisfactorily to a kinetic model describing the degradation as a combination of thermal scission, oxidation, and hydrolysis. PPD-T degradation is slower than that of MPD-I, and MPD-I resistance to degradation increases as polydispersity decreases. (C) 2003 Wiley Periodicals, Inc.