An analysis of the performance of a gas turbine-steam turbine combined cycle with supplementary firing has been carried out. Natural gas is fired in the main combustor of the cycle, whereas biomass fuel is considered as the supplementary fuel. Although, supplementary firing is found to reduce the overall cycle efficiency, the low cost of biomass and the CO2-neutral attribute of its combustion reduce the specific fuel cost and specific CO2 emission. The effects of pressure and temperature ratios of the topping cycle and main steam conditions of the bottoming cycle on the performance parameters of the combined cycle have been studied at different degrees of supplementary firing. The topping cycle temperature ratio is found to be the most critical parameter and its low value gives substantial advantages in lowering the fuel cost and CO2 emission. Marginal advantages are also achieved at higher pressure ratio and better bottoming cycle main steam conditions. Copyright (c) 2008 John Wiley & Sons, Ltd.