In this work, an industrial calcined Phillips CrOx/SiO2 catalyst engaged in the induction period through interaction with ethene at ambient conditions was characterized by temperature-programmed desorption (TPD) equipped with a quadrupole mass spectrometer (MS). It was unprecedented to find that the first hydrocarbon species had already been formed and was observed to be propene instead of the expected butene. In fact, butene is the second hydrocarbon species formed after propene during the induction period. This evidence strongly implied that the initiation in terms of Cr-carbon bond formation on the Phillips catalyst occurs through an ethene metathesis mechanism during the induction period. Three most possible routes regarding the ethene metathesis initiation and formation of the first hydrocarbon species propene during the induction period on the Phillips catalyst were proposed. (C) 2003 Elsevier Science B.V. All rights reserved.