An instrument to promote the CO2 emission reductions, taking the Kyoto Protocol goal into account, can be the assignment to energy conversion plants of a monetary charge linked to their specific emission intensity. Once the choice of a charge is defined, the next problem is the choice of a strategy to determine the amount of the imposed charge, named Carbon Tax (CT).In this paper an analytical procedure for the Carbon Tax evaluation is proposed and applied. This approach is based on the concept of Efficiency Penalty of the energy system, that represents the evaluation of the cost of the exergy destroyed inside the system and the cost of the exergy rejected in the biosphere with the plant wastes; the Efficiency Penalty term is coupled with the evaluation of the Index of CO2 Emission, which connects the amount of the CO2 emitted by the plant with the Second Law efficiency of the plant itself. The evaluated charge on the CO2 emissions is defined as Carbon Exergy Tax (CET). The procedure is applied here to the analysis of a 700 MW combined plant burning fossil fuels in two different configurations: a typical natural gas fired combined plant, and a coal fired combined plant burning coal in a Pressurised Fluidised Bed Combustor (PFBC).