Distributions of carboxy groups among the molecules in 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized cellulose nanofibrils (TOCNs) prepared from wood, cotton, and algal celluloses were investigated. Most C-6-carboxy groups in TOCNs were esterified with anthracenemethyl (-CH2C14H9) groups, showing an ultraviolet light (UV) absorption peak at 365 nm. The anthracene-methylated TOCNs were dissolved in 8% (w/w) lithium chloride/N,N-dimethylacetamide (LiCl/DMAc). After dilution to 1% LiCl/DMAc, the solutions were subjected to size-exclusion chromatography with multiangle laser-light scattering, refractive index, and UV detection. For algal TOCN, C-6-carboxy group-rich molecules were present predominantly in the low-molar-mass region, which was consistent with the core-clad cellulose chain packing structures in individual algal cellulose microfibrils and partial depolymerization of the oxidized cellulose molecules. In contrast, wood and cotton TOCNs had almost homogeneous distributions of C-6-carboxy groups in all molar mass regions, which could not be explained in terms of the simple core-clad cellulose chain packing structures.