The MoO rAl O catalytic systems were studied. The catalysts were highly selective and moderately active in propene 3 2 3
Ž . metathesis at a low temperature 303 K . It was shown by XPS that preliminary thermal treatment of the catalysts affects their final reactivity towards propene. The surface of the molybdena-alumina catalyst, which was evacuated at 523 K and then activated at 873 K in argon, underwent partial reduction to Mo V phase under the influence of propene at 303 K. This effect was not observed in the case of the molybdena-alumina catalyst, which was calcinated at 823 K and then analogously activated. It was concluded that the reducibility of the surface molybdenum was connected with the activity of the catalyst in propene metathesis. It was proposed that the difference of the reactivity towards propene of both catalysts was caused by the difference in the homogeneity of the surface molybdenum. It was stated that the activation of the catalysts in argon at 873 K increased the homogeneity of the surface molybdenum.