Promoting effects of Mg in heterogeneous Mo/HBeta-Al2O3 catalyst have been carefully studied for cross-metathesis of ethene and butene-2 to propene. The catalyst shows good stability with Mg content in the range of 1-2 wt%. Such effect may be attributed to the elimination of weak acid sites through introduction of Mg which suppresses the side olefin oligomerization reaction, as evidenced from NH3-TPD and H-1 MAS NMR results. Addition of more Mg content to 3 wt% may change the state and reducibility of Mo species, as indicated from the UV-vis. UV-Raman and H-2-TPR measurements. The increasing difficulty for the reduction of Mo(VI) species is closely related with the poor performance of 3 wt% Mg-4Mo/HBeta-30% Al2O3 catalyst in the metathesis reaction. (C) 2009 Elsevier B.V. All rights reserved.