Substitution of W for the Mo in M2-type Mo-V-Te-oxide was investigated. XRD, FTIR, XANES, and HRTEM verified that the substitution is possible in the whole range up to complete replacement of Mo by W. Catalytic measurements for propene ammoxidation show that both the specific activity and the selectivity to acrylonitrile pass through a maximum with increase of the W-content of the catalyst. XRD analysis indicates that the catalytic results, in part, can be explained by a disordered distribution of Mo, W, and V among the octahedral sites in M2 giving better catalytic performance than a more ordered arrangement does. Moreover, SEM and HRTEM imaging show that with increasing W-content, the M2 crystals change from mu m-sized elongated crystals to spherical crystals of nm-size, influencing the type of surfaces being exposed and consequently the catalytic properties. Among the catalysts investigated, a catalyst with the composition (Mo0.3W0.7)(1.9)V1.0Te1.1O10 is the most active and selective one, giving 80% selectivity to acrylonitrile. (c) 2007 Elsevier B.V. All rights reserved.