This work presents an investigation of the chemical and phase composition of Mo(1.0)V(0.3)Te(0.23)Ox catalyst prepared by spray drying. The changes in the catalyst after heat treatment at 220-550 degrees C were characterized by IR and Raman spectroscopy, XRD, HRTEM and differential dissolution methods. In the range of temperature 400-420 degrees C HRTEM revealed the presence of nanoscaled crystallites that are structurally and chemically related to M1 and M2 phases and randomly distributed in the amorphous matrix. The characteristic diffraction pattern of the amorphous product can be described by a model of nano-sized clusters built from the structure of M1 phase. Nanostructured catalysts are active and selective in propane ammoxidation. The increase of temperature of heat treatment of the catalyst above 450 degrees C leads to decomposition of nanostructured M1 phase and crystallization of M2 and a series of vanadium molybdenum phases. The process is accompanied by a decrease in activity. (c) 2014 Elsevier B.V. All rights reserved.