Extended periods of ionization of poly(p-phenylene terephthalamide) (PPTA), namely Kevlar 149, fibres by sodium methylsulfinyl carbanions leads to a significant deterioration in the fibre tensile properties. The non-chain-breaking nature of the ionization suggests that hydrogen bonds among the PPTA chains in the intercrystalline regions also contribute to the axial strength of the fibres. Rinsing with dimethylsulfoxide (DMSO) following the ionization initially improves the tensile strength. With DMSO immersions of 10 h or longer, the tensile strength loss resumes, and can be explained by hydrolytic chain scission of the PPTA molecules in the intercrystalline regions. Ionization which does not affect the fibre strength of Kevlar 49 is effective for structural differentiation between Kevlar 49 and 149 fibres. In a similar fashion to ionization, etching by argon glow discharge produces little change in Kevlar 49 fibre structure and strength. The fact that both ionization and glow discharge treatment affect PPTA fibre properties similarly demonstrates that both reactions depend on the accessibility and precise microstructure of these fibres.