This analysis addresses some aspects of the internal structure and extinction of diffusion flames established by burning of multicomponent fuel (n-heptane diffusion flame enriched with hydrogen). By presuming one-global-step reactions with large activation energy, the zone where all reactions take place is described by large-activation-energy asymptotics. In this study, it is also supposed that the values of the Damkohler numbers for the reactions are very close and the values of the activation energies are also very close. Under these conditions, all reactions will be performed inside the same region. The results show that the reaction with small Damkohler number and/or large activation energy controls the extinction of the multicomponent-fuel diffusion flame. Under the condition of equal Damkohler numbers and equal activation energies, Hamins and Seshadri's (Hamins, A. and Seshadri, K. (1984b) Proc. Combust. Instil., 20, 1905-1913) results are recovered.