MoVTeNb oxide catalysts exclusively composed of the M1 phase (ICSD No. 55097) have been studied in the direct oxidation of propane to acrylic acid applying a broad range of reaction conditions with respect to temperature (623-633-643-653-663 K), O-2 concentration in the feed (4.5-6.0-9.0-12.0%), steam concentration in the feed (0-10-20-40%), and contact time (0.06-0.12-0.18-0.24-0.36-0.48-0.72-1.44 s g(cat) Nml(-1)). The molar fraction of propane was kept at 3.0%. Model experiments were performed to study the reactivity of possible intermediates propene, acrolein, and CO. The impact of auxiliary steam on the chemical nature of the catalyst surface was analyzed by in situ photoelectron spectroscopy, while in situ X-ray diffraction has been carried out to explore the structural stability of the M1 phase under stoi-chiometric, oxidizing, and reducing reaction conditions. Phase purity apparently accomplishes absolute stability in terms of the crystalline bulk structure and the catalytic performance over month even under extreme reaction conditions. In contrast, the catalyst surface changes dynamically and reversibly when the feed composition is varied, but only in the outermost surface layer in a depth of around one nanometer. The addition of steam causes enrichment in V and Te on the surface at the expense of Mo. Surface vanadium becomes more oxidized in presence of steam. These changes correlate with the abundance of acrylic acid detected in the in situ photoelectron spectroscopy experiment. Analysis of the three-dimensional experimental parameter field measured in fixed bed reactors revealed that the complexity of the reaction network in propane oxidation over MoVTeNb oxide is reduced compared to less-defined catalysts due to phase purity and homogeneity. The oxidative dehydrogenation of propane to propene followed by allylic oxidation of propene comprises the main route to acrylic acid. The oxygen partial pressure was identified as an important process parameter that controls the activity in propane oxidation over phase-pure M1 without negative implications on the selectivity. High O-2 concentration in the feed keeps the catalyst in a high oxidation state, which provides an increased number of active sites for propane activation. Auxiliary steam increases activity and selectivity of M1 by changing the chemical nature of the active sites and by facilitating acrylic acid desorption. (C) 2014 Elsevier Inc. All rights reserved.