The experimental structure of a lean iso-butene/hydrogen/oxygen/argon flame (2.7% iC(4)H(8), 4.5% H(2), 83.0% O(2), 9.8% Ar, phi = 0.225) has been determined by molecular beam mass spectrometry at low pressure (40 mbar). The detected species throughout the flame thickness were: H(2), CH(3), O, OH, H(2)O, C(2)H(2), CO, C(2)H(4), CH(2)O, O(2), HO(2), Ar, C(3)H(4), C(3)H(6), CO(2), C(2)H(4)O, C(4)H(6), iC(4)H(8), C(3)H(6)O, C(4)H(6)O and C(4)H(8)O. An original model, validated against premixed rich C(2)H(4), has been extended by building a sub-mechanism taking into account the formation and the consumption of species involved in iso-butene combustion. This mechanism contains 520 reactions and 99 chemical species. A good agreement appears between calculated mole fraction profiles predicted by this mechanism, compared to experimental results. (C) 2010 Elsevier Ltd. All rights reserved.