—The non-similar weakly-reactive boundary layer flow over an isothermal flat plate is studied using a combined perturbation/numerical procedure. The solution yields an explicit ignition criterion which exhibits the proper functional dependence on all the system parameters of interest. Predictions on the minimum distance to achieve ignition agree well with results from complete numerical calculations. Reactant consumption is shown to be an important factor, especially for lean mixtures. Its inclusion enables the identification of an approximate expression for the absolute ignition limit of the system.