We report an FTIR spectroscopic technique combined with intracrystalline deuteration and rehydrogenation of cellulose samples to investigate the localization of I-alpha. and I-beta domains within a cellulose microfibril obtained from I-alpha-rich algae. When Glaucocystis cellulose incompletely converted from I-alpha to I-beta was deuterated and rehydrogenated at elevated temperature, OD groups involved in hydrogen bonding in the I-beta domain first reverted to OH, followed by those in the I-alpha domain, Suggesting that the I-alpha core domain was surrounded by the I-beta domain in an artificially induced sample. We concluded that this I-alpha -> I-beta conversion proceeded from the Surface toward the core. Native celluloses from Valonia and Cladophora were first deuterated without changing the allomorphic composition, and rehydrogenation was studied from I-alpha-and I-beta-specific absorbances. Surprisingly, both absorbances changed synchronously, clearly indicating that the simple "skin-core" distribution model of I-alpha and I-beta domains is not realistic at least for these native celluloses.