Different gases were employed to pretreat WO 3 /SiO 2 for the metathesis of 1-butene and ethene to propene. Air-pretreated WO 3 /SiO 2 was inactive, whereas N 2 -, N 2 /H 2 -, and H 2 -pretreated WO 3 /SiO 2 exhibited high 1-butene conversion and propene selectivity. Tetragonal WO 3 and partially reduced WO 2.92 were found to be the active phases/species, whereas monoclinic WO 3 was inactive. N 2 /H 2 - and H 2 -pretreated WO 3 /SiO 2 contained both W 6+ and W 5+ species (i.e., nonstoichiometric WO 3−x ). Air- and N 2 -pretreated WO 3 /SiO 2 only contained monoclinic W 6+ species (WO 3 ) before the reaction. However, the used N 2 -pretreated WO 3 /SiO 2 contained both W 6+ and W 5+ species, suggesting that the N 2 pretreatment can facilitate the formation of some W 5+ species during the reaction. This in situ partial reduction can activate the N 2 -pretreated WO 3 /SiO 2 for the metathesis.