Abstract. Microcrystalline cellulose is a porous natural material which can be used both as a support for nanoparticles and as a reducer of metal ions. Cellulose supported nanoparticles can act as catalysts in many reactions. Cu, CuO, and Cu2O particles were prepared in microcrystalline cellulose by adding a solution of copper salt to the insoluble cellulose matrix and by reducing the copper ions with several reducers. The porous nanocomposites were studied using anomalous small angle x-ray scattering (ASAXS), x-ray absorption spectroscopy, and x-ray diffraction. Reduction of Cu 2+ with cellulose in ammonium hydrate medium yielded crystalline CuO nanoparticles and the crystallite size was about 6 -20 nm irrespective of the copper concentration. The size distribution of the CuO particles was determined with ASAXS measurements and coincided with the crystallite sizes. Using sodium borohydrate or hydrazine sulfate as a reducer both metallic Cu and Cu2O nanoparticles were obtained and the crystallite size and the oxidation state depended on the amount of reducer.