Radioisotope labeling with 3H-label sodium borohydride was used in order to elucidate the the nature of reducing ends encountered by reducing end-specific-exo-acting cellobiohydroases. Model celluloses and soluble cellooligosaccharides were studied by the comparsion the apparent second order rate constants from the kinetics of tritium incorporation. The pattern of kinetic data comparison of the rate constants obtained for the incorporation of the different celluloses showed a difference between the reactivity of the reducing ends associated with amorphous versus microcrystalline celluloses. Reactivities of bacterial microcrystalline cellulose and microcrystalline cellulose were found to be significantly (p=0.0014, p=0.0018) lower than that of the reference cellooligosaccharides (cellotriose). However, there were no significant differences between the amorphous celluloses and cellotriose. Results of 3H labelling suggested that the terminal glucosyl units of the microcrystalline cellulose showed to be chemically distinct from those of their soluble cellooligosaccharide analogs.