Catalyst characteristics affecting product selectivity and mechanistic aspects of formation of Mo-carbene species in the metathesis of ethylene and trans-2-butene to propylene over MoOx/AlSiOx were elucidated by complementary techniques. Steady-state catalytic tests revealed that reaction pathways leading to propene formation depend on the kind (highly dispersed or polymerized) of MoOx species, from which Mo-carbene species are formed in situ. C3H6 is produced directly through metathesis of C2H4 and trans-2-C4H8 on Mo-carbenes originated from polymerized MoOx species, whereas the metathesis of land 2-butenes plays an important role for propene formation on Mo-carbenes formed from highly dispersed MoOx. In situ FTIR spectroscopy and kinetic analysis suggest that the pathways of propene formation and accordingly the selectivity are determined by the ability of C2H4 to react with oxidized MoOx species and Mo-carbenes. This feed component interacts with highly dispersed MoOx species more weakly than with polymerized ones. Moreover, C2H4 adsorbs more strongly on Mo-carbenes formed from highly dispersed MoOx than on those originating from polymerized MoOx species. (C) 2018 Elsevier Inc. All rights reserved.