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.