The effect of potassium and bismuth doping on the morphology and structure of M1 phase, which determines the activity of MoVTeNbO catalysts in the oxidative conversion of ethane to ethylene, was studied. According to XRD, HRTEM and XPS data, the introduced K and Bi enter the M1 phase and do not form individual compounds. The addition of potassium is accompanied by anisotropic growth of the M1 phase particles: an increase in potassium content enhances the growth of particles along the [001] direction. The insertion of bismuth does not change the morphology of M1 phase but produces structural features (extended defects, disorientation of the crystal structure) that increase the fraction of accessible surface formed by {001} faces. Along with the structural and morphological transformations, the both additives affect the ratio of V5+/V4+ and Mo6+/Mo5+ cations and the enrichment of the catalyst surface with tellurium. The observed changes in physicochemical characteristics of the promoted catalysts correlate with changes in the catalytic properties. (C) 2015 Elsevier B.V. All rights reserved.