Deactivation of (1) MoO3/Al2O3, (2) MoO3/Al2O3-Me4Sn, (3) Re2O7/Al2O3, and (4) Re2O7/Al2O3-Me4Sn catalysts preactivated in Ar in situ at 873 K was studied in the metathesis of propene run in a flow microreactor at 303 K, contact time 2.42, 12.1 or 484 kg(.)s/mol, for 60 min, whereupon C2H4, dry air, or Ar was passed for 10, 30 or 60 min, and then propene was added again to re-run metathesis for a total of 180-230 min. Catalyst load did not affect the reaction rate. For 2, air (O-2) was equiv. to Ar as deactivator; deactivation was intrinsic in nature. Ethene deactivated 1 and 2, but neither 3 nor 4; deactivation was partly reversi-=ble (with Me4Sn), related to time and rate of C2H4 addn. Toward 3, C2H4 was even favorable. Propene-dild. C2H2 deactivated 3 and 4. Polymn. of C2H4, C2H2 traces in C2H4, and reductive elimination of molybdocyclobutane complex centers were suggested as side effects operative in the deactivation.