A formulation is given for describing ignition in nonpremixed systems. Steady laminar how of two counterflowing streams toward a stagnation plane is considered. One stream comprises fuel and the other oxygen. The characteristic Reynolds numbers of the counterflowing streams are presumed to be large so that the thickness of the viscous layer formed in the vicinity of the stagnation plane is small. The chemical reaction between fuel and oxygen that takes place in the viscous layer is described by a one-step overall process. The activation energy of the reaction is presumed to be large in comparison to the thermal energy. The asymptotic theory developed here makes available explicit formulae for predicting ignition in the viscous layer. From these results a simple but reasonably accurate method is developed for deducing the activation energy, E, and frequency factor, B, of the rate of the one-step reaction between the fuel and oxygen. To illustrate the application of this method, experiments are carried out in the counterflow configuration. The fuels tested are n-heptane, n-decane, JP-10, and toluene. Experimental data obtained are the velocities and temperatures of counterflowing streams at ignition. Values of E and B are obtained by using the experimental data in the formulae given by the asymptotic theory. These values of E and B are found to agree well with those obtained from numerical calculations.