The activity of Bi2MoO6 catalysts with small amounts of excess bismuth in the selective oxidation of propylene is strongly dependent on the calcination conditions. These catalysts show reasonable activities and high selectivities after calcination for a short period of time or after calcination at a moderate temperature. However, these catalysts become low active and low selective after calcination for a long period of time or after high temperature calcination. This difference is ascribed to relative low Bi/Mo-ratios at (parts of) the surface after calcination for a short period of time. The active surface becomes enriched with bismuth during prolonged calcination or reaction, resulting in a deactivation of the catalyst. The deactivated catalysts can be reactivated by reduction, this supports that the deactivation is due to a surface enrichment with bismuth. The dependence of the catalytic activity on the calcination conditions is likely one of the reasons behind the conflicting results on the activity of Bi2MoO6 found in the literature. (c) 2006 Elsevier B.V. All rights reserved.