Molybdenum carbide associated with a surface dealuminated H-MFI appeared to be more active and more stable than when associated to the parent H-MFI, contrary to the expectation, in the conversion of a 1:1 mixture of methane and argon at atmospheric presssure and low flow rate (HPLF experiment). By contrast, under a low methane partial pressure and a high flow rate (LPHF experiment), the expected order of activity was found; the carbide associated with the parent zeolite showed a significantly higher activity. The discrepancy shown by the results of the two sets of experiments was interpreted in terms of a more rapid deactivation of the most active catalyst under a high methane load, resulting in a lower apparent activity. The higher stability of the carbide associated with the surface dealuminated zeolite resulted from the elimination of the surface sites which produce bulky aromatics with low vapor pressure leading to a rapid clogging of the pore mouths of the zeolite, in line with the general background on zeolite catalysis. (C) 2004 Elsevier Inc. All rights reserved.