Ethylene has selectively been obtained during the oxidative dehydrogenation of ethane on a Mo-V-Te-Nb-O mixed oxide catalyst. The catalyst was prepared hydrothermally and heat-treated at 600degreesC for 2 h in a N-2-stream. For comparison, undoped and Te-doped Mo-V-Nb-O catalysts and Mo-V-Te-O and Mo-Te-Nb-O mixed oxides have also been prepared. The catalytic performance of the Mo-V-Nb-Te-O mixed oxide catalyst cannot be explained by the incorporation of Te on the surface of a Mo-V-Nb-O mixed oxide but by the presence of an orthorhombic Te2M20O57 (M = Mo, V, Nb) crystalline phase. The role of tellurium in these catalysts is also discussed. (C) 2004 Elsevier B.V. All rights reserved.