The activation of a Mo9V3W1.2Ox catalyst was investigated in the partial oxidation of acrolein as function of reaction temperature and atmosphere. The activity and selectivity to acrylic acid considerably increased during activation in the acrolein oxidation reaction comparable to the recently reported activation after thermal pretreatment in inert gas. The activation during the catalytic acrolein oxidation, however, proceeds at about 200 K lower temperatures as compared to the inert gas pretreatment. The initial nanocrystalline catalyst structure changed during operation in the acrolein oxidation, as shown by X-ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy and energy dispersive X-ray (EDX) analysis. A (MoVW)(5)O-14-type mixed oxide was found to be the main crystalline phase in the active and selective catalysts. Hence, this (MoVW)(5)O-14 phase crystallizes already during catalysis at the low acrolein oxidation reaction temperatures. The evolution of the catalytic performance is directly related to this low temperature formation of the (MoVW)(5)O-14 phase. It is suggested that this (MoVW)(5)O-14 phase has to be present in a detected, specific ordering state which is vital for optimum selective oxidation properties. In addition, other minority phases were identified by transmission electron microscopy (TEM) in the operating catalyst. The so-called bundle-type, and a new corona-type texture was detected, which show ordering in only one or two dimensions, respectively. These disordered structures are also relevant candidates for active catalyst phases as they are detected during the activation period of the MoVW mixed oxide. (C) 2002 Elsevier Science B.V. All rights reserved.