A new method is proposed to reduce complex kinetic mechanisms to a few elementary reaction steps which predict accurately ignitions and extinctions of spatially distributed systems : This method combines principal component analysis with sensitivity analysis and is applied to the stability of premixed hydrogen/air flames near surfaces using numerical bifurcation theory. The full mechanism of 40 reactions of hydrogen/air flames is reduced to various elementary mechanisms which can predict ignition and extinction temperatures at atmospheric pressure with a maximum error of similar to 0.3 and 3%; respectively. The quasi-steady-stale approximation of all species near critical points is examined, and the mechanisms leading to instabilities are discussed as a function of reactor pressure. This analysis leads to only three reactions whose rates are linear combinations of five elementary reactions. It is shown that some fast reactions do not affect critical points. In addition, elimination of slow initiation reactions results in change of an ignition from a turning point to a transcritical bifurcation point. Analytical criteria for ignition and extinction are also proposed. Using these criteria, the accuracy in estimating kinetic parameters from experimental bifurcation data is assessed.