A thermo-economic method for the promotion of the efficient use of exergy resources is proposed, named carbon exergy tax (CET). It is based on the evaluation of the cost of the exergy destroyed and rejected, and connected with the CO2 emissions of an energy system. The CET and the carbon tax (CT) formulations are both applied to different conventional energy systems, to show how the CET strongly improves the economic suitability of systems which make a better use of exergy. Finally, the CET contribution to the widespread utilization of advanced energy systems (with the example of a solid oxide fuel cell + gas turbine plant) is presented and discussed. (C) 2002 Elsevier Ltd. All rights reserved.