A systematic approach for on-the-fly mechanism reduction is proposed in this paper. The approach consists of element flux analysis and dynamic generation of reduced mechanisms during reactive flow computation. At each time step of the computation, a reduced mechanism is generated based on local conditions and no previous information or analysis is necessary. The approach is demonstrated in a plug-flow reactor (PFR) model and a pairwise mixing stirred reactor (PMSR) model. A detailed n-pentane oxidation mechanism with 385 species and 1895 elementary reactions and a primary reference fuel (PRF) mechanism with 1034 species and 4236 reactions are used illustrate the proposed approach. Auto-ignition delay, temperature, and species composition profile predicted by the on-the-fly reduction scheme agree with the detailed simulation and experimental results over a wide range of temperatures and equivalence ratios. (C) 2009 Elsevier Ltd. All rights reserved.