In a series of experiments, three cylindrical premixed combustors have been tested to investigate the processes leading to NOx formation during the combustion of natural gas at gas turbine conditions. The variation of NOx emission levels with airfactor, combustor wail temperature, combustion power and various flameholder types were investigated. The results confirm that Nox emissions at lean conditions are strongly dependent on airfactor and independent of flameholder geometry. The NOx emissions can be correlated with the adiabatic flame temperature. The correlation of Nox with adiabatic flame temperature is however dependent on the combustor size. This can be ascribed to different relative heat losses. Therefore attention to flame heat transfer is important for predicting NOx emissions in the computational combustion modelling. The NOx emissions are less sensitive to the combustor wall temperature. Hence correct prediction of flame radiation is essential. No CO emission is found in these experiments. The prediction of NO in Ommen G-gas (natural gas, Groningen, The Netherlands) has been calculated in the perfectly stirred reactor (PSR) using Warnatz's and Correa's mechanism. The predicted results have been compared to the measured data.