The reaction mechanism of the direct conversion of ethylene to propene over Ni-containing catalysts is assumed in literature to comprise (i) ethylene dimerization to 1-butene, (ii) butene isomerization and (iii) metathesis of 2-butene with ethylene to propene. To elucidate if the third step of this reaction is really metathesis, the reaction of C-13-labelled ethylene with C-12-trans-2-butene was studied and the distribution of C-12 and C-13 atoms in propene and the other reaction products was measured by a gas chromatograph with mass spectrometer detector. Since only traces of propene containing one C-13 atom were observed, metathesis cannot play a substantial role in propene formation. This conclusion was confirmed by the lack of other products that would have been formed in secondary reactions by metathesis. The observed reaction products and the distribution of C-12 and C-13 atoms therein strongly suggest a reaction mechanism involving C-C olefin coupling reactions at Ni2+ surface sites and cracking reactions of the formed higher olefins at acidic sites of the catalysts. The proposed mechanistic pathway explains also the differences in the distribution of C-12 and C-13 atoms in propene formed over the two catalysts differing in their nature of Ni2+ surface species and acidic sites. (C) 2020 Elsevier Inc. All rights reserved.