Te-free and Te-containing Mo-V-Nb mixed oxide catalysts were diluted with several metal oxides (SiO2, gamma-Al2O3, alpha-Al2O3, Nb2O5, or ZrO2), characterized, and tested in the oxidation of ethane and propane. Bulk and diluted Mo-V-Nb-Te catalysts exhibited high selectivity to ethylene (up to 96%) at ethane conversions <10%, whereas the corresponding Te-free catalysts exhibited lower selectivity to ethylene. The selectivity to ethylene decreased with the ethane conversion, with this effect depending strongly on the diluter and the catalyst composition. For propane oxidation, the presence of diluter exerted a negative effect on catalytic performance (decreasing the formation of acrylic acid), and alpha-Al2O3 can be considered only a relatively efficient diluter. The higher or lower interaction between diluter and active-phase precursors, promoting or hindering an unfavorable formation of the active and selective crystalline phase [i.e., Te2M20O57 (M = Mo, V, and Nb)], determines the catalytic performance of these materials. (C) 2007 Elsevier Inc. All rights reserved.