Multi-step, gas-phase chemical kinetics are introduced into flame spread modeling efforts. An unsteady multi-step, gas-phase kinetics model both with and without steady-state species assumptions, and including nonunit Lewis number, is compared with a model including a single, finite-rate gas-phase reaction, which has been the usual approach in flame spread modeling. Laminar diffusion flames over a thin fuel in an opposing O-2/N-2 flow are considered with the solution in two-dimensional space of momentum, energy, and 12 gas-phase species. Results for the multi-step models show detailed flame structure in terms of species and heat release distributions throughout the flame and the role of chemical kinetics as a controlling mechanism in flame spread. Of particular interest is the potential of multi-step chemical kinetics in solutions at near-extinction limit conditions. While the incorporation of nonunit Le alone affords more detailed species transport, in high opposing hows it was found to give only minor structural differences from the single-step unit Le model. The multi-step chemistry allows for the gas kinetics to be self-adjusting to environmental conditions. As a result, the distribution of endothermicity and exothermicity throughout the flame and for particular reversible reactions is found to be a function of the flow environment, which overcomes a major drawback of single-step models, namely a fixed heat of combustion independent of environmental conditions, or one that must be determined separately from the model itself.