The aim of this work is to improve the knowledge of modeling NO, formation in hydrogen flames. Four different detailed reaction mechanisms have been tested for eight laminar flames, and two of these mechanisms have been tested for a turbulent jet flame. The numerical results have been compared with experimental data from the literature. Sensitivity and integral reaction flow analyses are applied to identify important reaction steps. Formation of NO through NNH radicals was found to be important in the hydrogen-air flames investigated. This work suggests that the H-2/O-2 mechanism of Li et al. for pure hydrogen combustion may be combined with the N/H/O subset from Glarborg et al. for prediction of NOx in hydrogen-air flames. However, the pressure-dependency of the reaction N2O + M reversible arrow N-2 + O + M should be further investigated and accounted for. For the turbulent hydrogen jet flame, the agreement between the predicted and measured NO levels was better with the mechanism of Glarborg et al. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.