Alumina, 210 m(2)/g, impregnated with aq. (NH4)(6)-Mo7O24.4H(2)O to yield a 10% MoO3, catalyst and dried 4 h/383 K, was (A) heated 4 h [or (A(16)), 16 h] in lab. air at 800 Pa, 523 K, or (B) calcined 4 h in dry air at 1 atm, 823 K, to yield, upon activation (Ar, 2 h, 873 K), sp. surfaces of 196 and 204 m(2)/g. resp. The catalysts A and B were tested in a 5-h metathesis of propene (20 mL/min, 36.3 kg.s/mol, 303 K); the A was flushed with Ar at 303 K, regenerated (air, 4 h, 800 Pa, 523 K) and reactivated (Ar, 2 h, 873 K); metathesis was re-run 3 h and the A was heated (air, 4 h, 523 K, 800 Pa) and reactivated (Ar, 2 h, 873 K), metathesis was still re-run (5 h) twice with due regenerations of A and B. The B was regenerated (dry air, 4 h, 823 K), the test was repeated twice and the subsequent regenerations took 4 and 1 h. The A and B were activated 2 h in Ar at 873 K and impregnated at 303 K with Me4Sn (Sn:Mo = 1:20 by moles) in pentane, tested in the metathesis (60 mL/min, 12.1 kg.s/mol, 303 K), and regenerated (air, 4 h, 800 Pa, 523 K) also by adding Me4Sn (0.05 mol/mol Mo in catalyst), followed by standard thermal activation in situ. For the A and B, repeated thermal regenerations were effective; for MoA(or B)-Sn(CH3)(4), thermal retreatment had to be augmented by re-addition of activator.