This paper presents experimentally measured data showing the impact of variable demand on a modern 800 MW CCGT plant. The results contrasting the performance of the plant when operating under optimum conditions with those measured when modulating the output to match dispatch instructions is presented and compared. These contrasts include the impact of step changes, continual modulation and both hot and cold starts of the plant. The results indicate the changes in fuel used per MWh, CO2 emitted per MWh and the NOx emissions under different operating modes. From the subsequent analysis significant increases were recorded in both fuel used and CO2 emitted when the plant departs from optimum operating conditions. When the plant is requested to cease generating due to over capacity on the system, major increases in the emissions of NOx, when required to restart generation together with large increases in the fuel used and CO2 emitted per MWh, can be observed. (C) 2010 Elsevier Ltd. An rights reserved.