The concern over 'global warming' and 'acid rain', stands out the possibility of re-converting fuel-oil boilers into natural gas, because SO2 emissions would be almost annulled but also because it would mean a great decreasing of 'carbon' in the fuel matrix, reducing also CO2 emissions. Another important fact is the progressive substitution of conventional power plant schemes for combined cycles, due mainly to their higher efficiency. In this paper, a comparison (in terms of emissions) was carried out between both cycles for natural gas and fuel-oil consumption over a representative Spanish thermal power plant. Finally, a comparison with experimental values has been performed. (c) 2006 Elsevier Ltd. All rights reserved.