The change in strength characteristics of poly(naphthoyleneimidobenzimidazole) films and fibers during aging in air at room temperature was shown to be accompanied by the slow degradation of the macromolecules. The degradation was proven by the fact that in concentrated sulfuric acid the polymer exhibited lower intrinsic viscosity, a greater translational diffusion coefficient, and a smaller characteristic orientation in flow birefringence experiments. The decrease in molecular mass was greatly accelerated when poly(naphthoyleneimidobenzimidazole) was on the powder state. Resynthesis (heating in vacuum at 200°C) resulted in partial recovery of the molecular mass; however, both resynthesized and reprecipitated (into the water) samples retained the trend toward self-degradation.