This paper presents numerical simulation results from the modelling of a turbulent non-premixed hydrogen (H-2) and hydrogen-hydrocarbon flame with and without radiation models. CFD studies using Fluent code were carried out for three cases; without radiation model, with the P-1 radiation model and with the discrete transfer radiation model. The model results from these three cases are compared with each other and with the experimental results. The effects of fuel composition from pure natural gas to hydrogen (100% CH4, 70% H2 + 30% CH4, and 100% H-2) were also investigated. The predictions are validated and compared against the experimental results obtained in this study and results from the literature. Turbulent diffusion flames are investigated numerically using a finite volume method for the solution of the conservation equations and reaction equations governing the problem. The standard k-e model was used for the modelling of the turbulence phenomena in the combustor. The chemical combustion reactions are described by seven species and three steps. A NOx post-processor has been used for predicting NOx emissions from the combustor. Using the both radiation models caused over all lower temperature levels for all fuel compositions compared with the results obtained using no radiation model. The results with the radiation models are in better agreement with the measurements compared with the results without radiation model. The use of a radiation model predicts lower over all temperature, thus lower NOx emissions. (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.