Transition metal (Cu Co Ni and Zn)-modified cryptomelane-type manganese dioxide nanomaterials were synthesized by the milling method The obtained solids have been characterized by means of Xray diffraction (XRD) scanning electron microscopy and transmission electron microscopy (SEM and TEM) N(2) adsorption-desorption measurements at 77 K Raman spectroscopy and temperature programmed reduction (TPR-H(2)) showing similar structural and textural properties All the solids were active in the preferential oxidation of CO in the presence of hydrogen (PROX) being the modified with copper the most active The catalytic activity correlates fairly well with the TPR results finding higher CO conversion for the material with higher reducibility (OMS-Cu) The O(2) selectivity measured as ([CO](in)-[CO](out)/2[O(2)](in)-[O(2)](out)) x 100 is very similar for all synthesized materials (C) 2010 Elsevier B V All rights reserved