Because it represents a polymer system routinely used in a highly corrosive environment, the resin-bonded cellulose separator is a paradigm of bad polymer application that succeeds. It succeeds because of the remarkable properties of cellulose itself, and through the formation of a highly modified structure that serves to protect both the resin and the cellulose in a reactive environment known to completely degrade both. X-ray diffraction analysis was used to determine the quality of retained cellulose in lead–acid battery separators made by impregnating thick cellulose matrixes with phenolic resins. The hydrolytic weight loss of separators stored in battery acid at 52°C was also measured and related to x-ray diffraction measurements of intensive degree of crystallinity and “qualitative perfection.” The x-ray diffraction analyses and hydrolysis measurements are consistent with the formation of a copolymer between the cellulose and the impregnating resin. Treated and rewetted α-cellulose displays a capability for reorganization, indicated by increases in intensive crystallinity and qualitative perfection that are absent in the resin-bonded cellulose separator matrix. Rewetting of the separator matrix actually decreased the “qualitative perfection” of the cellulose present. X-ray diffraction measurements indicate that extensive crystallinity of acid-treated separators increases, due probably to the higher reactivity of the “so-called” amorphous fraction of the cellulose present. The degree of crystallinity, an intensive measure, does not change significantly during acid treatment, but the “qualitative perfection” of the cellulose decreases with time of hydrolysis and increases in acid strength.