The effect of mercerization at elevated temperatures upon cotton fiber substructure has been studied by electron microscopic techniques. Although some advantages can be attained by raising the temperature of the caustic solution, fiber substructure exhibits anomalous behavior. Photomicrographs of fibers from yarns treated at the extreme temperature ranges reveal a skin-core appearance. The conversion of cellulose I to cellulose II crystalline structure normally associated with mercerization is retarded at elevated temperatures as shown by x-ray analysis. This paper was presented at the National ACS meeting 10-15 September 1978. -- AATA