MoVTeNbO catalysts, prepared by hydrothermal synthesis, are active and highly selective in the ODH of ethane, especially those with a Mo-V-Te-Nb molar ratio of 1-0.15-0.16-0.17 and heat-treated at 600-650 degreesC. On the best catalyst, selectivities higher than 80% at ethane conversion levels higher than 80% have been obtained operating at relatively low reaction temperatures (340-400 degreesC). Thus, yields of ethylene of ca. 75% have been obtained, which exceeds the best yield reported in the literature. Te2M20O57 (M = Mo, V, Nb) and (V,Nb)-substituted theta-Mo5O14, in addition to small amounts of the Te0.33Mo3.33 (M = Mo, V, Nb) phase, can be proposed in the most selective catalysts from XRD, SAED, and HREM results. However, the catalytic performance in ethane oxidation can mainly be related to the presence of the multifunctional Te2M20O57 orthorhombic phase in cooperation with the Mo5O14-type phase. The nature of active and selective sites is discussed. (C) 2004 Elsevier Inc. All rights reserved.