A numerical investigation of the structure of hydrogen/air partially premixed flames (PPFs) in a counterflow configuration is reported. The partially premixed flame structure is characterized by two spatially distinct reaction zones. Unlike hydrocarbon/air PPFs, the fuel is partially consumed in the rich premixed zone, where H, 0, and OH are the major intermediate species, with the remaining fuel being consumed in the nonpremixed zone. In both the reaction zones, consumption of reactants occur primarily through reactions H + O-2 <----> OH + O (R 1), H-2 + O <----> OH + H (R2), H-2 + OH <----> H2O + H (R3), and H + O-2 + M <----> HO2 + M (R9). Maximum heat release occurs in the rich premixed zone through reactions R9 and R3. Interactions between the two reaction zones involve the transport of heat from the nonpremixed to the premixed reaction zone and the transport of H, from the premixed to the nonpremixed zone. The flame response to variations in equivalence ratio (0) and strain rate (a,) is characterized. Increasing phi and/or a(s) causes the two reaction zones move closer, and eventually merge with each other. Further increase in phi leads to a nonpremixed flame, while that in a(s) leads to stretch-induced extinction. Finally, differences between the structures of hydrogen PPFs and hydrocarbon-fuel PPFs are highlighted. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.