The structures of different celluloses (microcrystalline and native) were characterized by diffuse reflectance infrared spectroscopy (DRIFT), analyzing the bands due to O-H stretching, C-O stretching, and C-H bending modes. The respective crystallinities were determined as 73 +/- 2% and 40 +/- 2%, and the crystalline structure in both was characterized as type I beta. A comparative study of adsorption of an aromatic ketone, benzophenone, on these two substrates was made, using the same technique. Through the modifications observed in the carbonyl stretching band of benzophenone, it was possible to distinguish adsorption in three different environments : entrapped between the glycosidic chains in crystalline domains, in amorphous domains, and as crystallites adsorbed at the cellulose surface. By deconvolution of spectra and subsequent analysis of the main components’ relative intensities, it has been possible to calculate the approximate amounts of entrapped molecules per gram of cellulose as a function of total benzophenone concentration. These results show that, for low benzophenone concentrations, adsorption occurs preferentially in crystalline domains of cellulose, independently from its crystallinity. On the contrary, as concentration is increased, crystallinity has a determinant role on the total amount of adsorbed benzophenone and on its molecular distribution in the different domains of cellulose.