Unusual fractions of cellulose microfibrils from woody material with dimensions of hundreds of nanometers in length and single digit angstrom thickness were obtained by intensive sonication of TEMPO-oxidized cellulose fibers. These cellulose microfibril fragments, composed of many mono- and bilayer molecular sheets, were analyzed with scattering and spectroscopy techniques to understand the structural changes at the supramolecular level. XRD data indicated that sonication breaks the cellulose microfibrils along its (200) planes, yet some form of the Iβ crystalline structure is still retained with reduced crystallinity. The Raman and FTIR analysis indicated structural changes to the cellulose microfibrils do not occur until after sonication; furthermore, AFM observation indicates that the structural changes began to occur within 5 min of sonication. An altered supramolecular structure is evident after sonication: major features from cellulose I are preserved, although certain spectral features similar to mercerized and ball milled cellulose appeared in its FTIR and Raman spectra. These spectral differences are traced to changes in the methine environment, hydroxymethyl conformations, and skeletal vibrations. By integrating the present findings and previous research, a cellulose molecular sheet delamination scheme is proposed to describe this microfibril fragmentation along its (200) plane.