The first step of the catalytic cycle of ethene metathesis on molybdena-alumina catalyst, i.e., ethene addition to surface Mo methylidene centres, is investigated by using the paired interacting orbitals (PIO) analysis based on the extended Mickel method. It is shown that the PIO theory is an adequate approach for both qualitative and quantitative description of the process. The total overlap population between the Mo methylidene part and the C2H4 fragment of the Mo ethene-methylidene complex decreases with a decrease of the reactivity of the Mo methylidene centre towards ethene. Therefore, the total overlap population is expected to be a useful reactivity index for the molybdenum-based olefin metathesis catalysts. (C) 2008 Elsevier B.V. All rights reserved.