This study presents a simple and quantitative scheme to determine the active sites in chemical reactions. We first introduce a novel index for characterizing the potential energy profiles in the reactions. Next, the energy density analysis is applied to expand the energy differences of the reactant or the product with respect to the transition state. By setting the threshold for the differences, one can extract the active sites, in which greater energetic changes happen in the reactions. Numerical applications, performed for the C-H activation of methane by a Ti(IV)-imido complex, have confirmed the usefulness of the present scheme. (C) 2008 Elsevier B.V. All rights reserved.