Four different cluster models of Mo(VI) methylidene centres in the HZSM-5 framework are investigated with DFT. The positive charge on the MoO(CH2) fragment is higher for the Mo centre with two Al atoms in its vicinity, compared to the models with only one Al site. The activity of two selected Mo centres is examined by the calculation of the pathway of ethene metathesis. In both cases the cycloaddition of ethene to the Mo methylidene centre proceeds easily. On the other hand, the cycloreverse step involves a relatively high energy barrier for the Mo centre with one Al site in the vicinity, whereas the barrier is low for the model with two T sites occupied by Al atoms. Moreover, the key molybdacyclobutane intermediate is a thermodynamically stable species in the first case, but it is unstable in the second case. Thus, a high metathesis activity is predicted if two Bronsted acid sites are replaced by the Mo centre. A secondary molybdacyclobutane structure can be formed as well, but this species is less stable than the primary metallacycle. (C) 2009 Elsevier B.V. All rights reserved.