It has been shown that the changes with time of the mechanical characteristics of films and fibres of poly(naphthoyleneimidobenzimidazole) (PNIB) under atmospheric conditions at room temperature are accompanied by slow degradation of the macromolecules. This is indicated by a drop in intrinsic viscosity, an increase in translational diffusion of the macromolecules and a decrease in characteristic orientation of flow birefringence (FB) in PNIB solution in concentrated H2SO4. The decrease in molecular (M) is considerably accelerated for PMIB in the powdered state. Resynthesis (heating under vacuum at 250°C) partially restores M of the sample. However, both resynthesized and precipitated (into water) PNIB samples retain a tendency to self-degradation. © 1994.