The passivation and reactivation of Mo2C and W2C were investigated by thermogravimetry with effluent gas analysis, and by comparison of the toluene hydrogenation activity of fresh carbides with those of passivated' and reactivated carbides (21 bar, H-2:toluene = 33, 150 or 200 degrees C, WHSV = 10 or 20 g g(cat)(-1) h(-1)). Contrary to the literature, CO2 and H2O were unsuitable passivation agents for Mo2C. Mo2C and W2C reacted readily with 02 at 40 C. The mass gained during oxidation could be quantitatively removed through reduction in 1 bar H-2 at 300 degrees C (Mo2C) or 400 degrees C (W2C). The rate of toluene conversion was fully recovered only for Mo2C that had been passivated with an initial 02 concentration of 0.1 vol.% and not for other passivation conditions or for W2C. Intervening operation of toluene hydrogenation at 300 or 400 degrees C enhanced the activity of samples that were previously oxidized and reduced or harshly reduced. This enhancement suggests that carbide formation is possible at milder temperatures than typically employed. (C) 2017 Elsevier Inc. All rights reserved.