The development is described of a simple lumped parameter NOx model suitable for on-line control of pollutant emissions. This NOx model has the potential to deal with a wide range of operational conditions and was developed for a coal-fired power station in Northern Ireland. The plant is equipped with low NOx burners, requiring the modelling of combustion and NOx reactions in both fuel-rich and fuel-lean combustion zones. Both volatile and char-based NO. are included, with fuel NOx formation and reduction modelled in the fuel-rich region, whereas char NOx and thermal NOx were modelled in the outer fuel-lean zone. Model constants were determined by parameter estimation using existing plant data. The predicted NOx output compared favourably with actual plant emissions.