The attractive features of a combined cycle (CC) power plant are fuel flexibility, operational flexibility, higher efficiency and low emissions. The performance of five gas turbine-steam turbine (GT-ST) combined cycle power plants (four natural gas based plants and one biomass based plant) have been studied and the degree of augmentation has been compared. They are (i) combined cycle with natural gas (CC-NG), (ii) combined cycle with water injection (CC-WI), (iii) combined cycle with steam injection (CC-SI), (iv) combined cycle with supplementary firing (CC-SF) and (v) combined cycle with biomass gasification (CC-BM). The plant performance and CO2 emissions are compared with a change in compressor pressure ratio and gas turbine inlet temperature (GTIT). The optimum pressure ratio for compressor is selected from maximum efficiency condition. The specific power, thermal efficiency and CO2 emissions of augmented power plants are compared with the CC-NG power plant at the individual optimized pressure ratios in place of a common pressure ratio. The results show that the optimum pressure ratio is increased with water injection, steam injection, supplementary firing and biomass gasification. The specific power is increased in all the plants with a loss in thermal efficiency and rise in CO2 emissions compared to CC-NG plant. Copyright (C) 2013 John Wiley & Sons, Ltd.