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Abstract: New insights into the application of One-Machine Infinite Bus (OMIB) equivalents to the analysis and study of the inter-area mode phenomenon in stressed power systems are provided. First, the notion of a time-varying OMIB equivalent is briefly reviewed and details of the adopted analytical model are introduced. On the basis of this representation, a systematic technique is then proposed to extract the underlying dynamics of the process directly from time-domain transient stability simulations. The analysis tool takes into account both, the dynamics of the slow inter-area oscillations and the dynamics of the fast inter-machine oscillations. Criteria to interpret inter-area oscillations in terms of the kinetic energy transfer between the most disturbed machines, and the rest of the system following the onset of a major contingency are given and methods to implement the proposed approach are discussed. A case study on a 5-area dynamic equivalent model of the Mexican interconnected system in which several static VAR compensators are used to support system voltage is presented to illustrate the developed procedures. Studies are conducted to identify and characterize the energy transfer process that accompanies the inter-area mode separation as well as to evaluate the effect of voltage support on system damping. The results show that time-varying OMIB dynamic equivalents can be used to capture the dominant features of the inter-area mode behavior, which are needed in the analysis and control of system dynamic performance. [Copyright &y& Elsevier]