Poly(m-phenylenediamineisophthyl)amide (MPD-I) is a polymer that is particularly well suited for the manufacture of filters, due to its high chemical and thermal resistance as well as processability characteristics. The degradation of this polymer was studied, both as fibers and as a filter cloth, in a proprietary laboratory degradation chamber and in an industrial environment. The results were fitted to a kinetic model describing the degradation as a combination of thermal scission, oxidation, and hydrolysis. The model fits well with the experimental results, and the laboratory induced degradation adequately simulates the conditions in which polymers are used industrially.