The shape and size distribution of crystalline nanoparticles resulting from the sulfuric acid hydrolysis of cellulose from cotton, Avicel, and tunicate were investigated using transmission electron microscopy (TEM) and atomic force microscopy (AFM) as well as small- and wide-angle X-ray scattering (SAXS and WAXS). Images of negatively stained and cryo-TEM specimens showed that the majority of cellulose particles were flat objects constituted by elementary crystallites whose lateral adhesion was resistant against hydrolysis and sonication treatments. Moreover, tunicin whiskers were described as twisted ribbons with an estimated pitch of 2.4-3.2 mu m. Length and width distributions of all samples were generally well described by log-normal functions, with the exception of tunicin, which had less lateral aggregation. AFM observation confirmed that the thickness of the nanocrystals was almost constant for a given origin and corresponded to the crystallite size measured from peak broadening in WAXS spectra. Experimental SAXS profiles were numerically simulated, combining the dimensions and size distribution functions determined by the various techniques.