Dissolving-grade wood pulp fibers were partially esterified by mixed p-toluene sulfonic/hexanoic acid anhydride in a nonswelling suspending agent. A biphasic morphology was revealed by atomic force microscopy (AFM) for the compression-molded, partially modified pulp fibers. The AFM phase images indicated distinct periodicity on the scale of several 10's of nanometers. Surface etching with cellulolytic enzymes of the modified pulp fibers produced height images that had virtually the same periodicity. These results indicate that the modified pulp fibers are nanocomposites comprising unmodified cellulose and cellulose hexanoate. Regenerated lyocell fibers (from N,MMNO solvent) subjected to the same esterification system as applied to pulp fibers, by contrast, exhibited AFM phase images that indicated a high level of surface (skin) versus core reactivity. Modified lyocell fibers with an average diameter of about 12 mum and having an overall DS of 0.6 had surface layers that were approximately 1 mum thick. The latter represented a transitional phase in which the chemical composition and the physical properties were intermediate between a highly substituted surface (skin) and an unsubstituted core. When a compression-molded sheet of the modified lyocell fibers was analyzed by microthermal analysis, the thermoplastic matrix on the lyocell fiber surface was revealed to have an apparent T-g of 75 degreesC corresponding to cellulose hexanoate, whereas no significant thermal transition was determined for the (unmodified) fiber core. These results suggest that both partially modified lyocell fibers and partially modified pulp fibers are capable of producing composites with morphologies that have grossly different scales.