Repowering fossil fuel power plants by means of gas turbines has been traditionally considered to increase power output and reduce NOx and SO2 emissions both at low cost and short outage periods. At present, reduction in CO2 emissions represents an additional advantage of repowering due to partial fuel shift and overall efficiency increase. This is especially important in existing installations with a CO2 reduction mandatory that should be carried out in a short time and in a cost-effective manner. Feedwater and parallel repowering schemes have been analysed using thermodynamic, environmental and economic simulations. The objective is not only to evaluate the cost of electricity and the efficiency increase of the overall system, but calculate and minimize the cost of CO2 avoided as a function of gas turbine power output. It seems that integration of larger gas turbines reduces the overall CO2 emissions, but there is a compromise between CO2 reduction due to fuel shift and a optimum integration of waste heat into the power plant to minimize the CO2 avoided costs. Results highlight the repowering as a suitable technology to reduce 10-30% of CO2 emissions in existing power plants with cost well below 20 (sic)/tCO(2). it could help to control emissions up to the carbon Capture technologies commercial development. (C) 2009 Elsevier Ltd. All rights reserved.