A series of Mo-MCM-41 mesoporous molecular sieves of varying Mo content were synthesized and the local structures of their active sites characterized by means of XRD, UV-vis, photoluminescence, and XAFS measurements. Mo-MCM-41 was found to possess three different types of Mo-oxide species, the concentrations of which were dependent on the Mo content: a highly dispersed tetra-coordinated monomeric molybdate, a tetra-coordinated dimeric and/or oligomeric molybdate, and a hexa-coordinated polymeric molybdate. Moreover, the tetra-coordinated molybdate species was observed to play a major role in the photocatalytic reduction of NO in the presence of CO to form N-2 and CO2. (c) 2005 Elsevier Inc. All rights reserved.