This paper studies chemically reacting, spatially evolving, supersonic mixing layers by performing large eddy simulations. Specifically, the goal is to reproduce the experimental results on molecular mixing and heat release performed at Caltech by Bonanos et al.1 Here, the mixing layer is formed as a result of the interaction of a supersonic stream and a subsonic stream. The supersonic stream expands over a 30° ramp and interacts with a subsonic stream of fluid injected into the combustor through the ramp. The primary (top) stream contains a small amount of H2 as the fuel. The secondary stream (injection through the ramp) contains a fractional amount of F2 which acts as the oxidizer. The hypergolic reaction between hydrogen and fluorine gives a large value of Damkohler number, which makes the chemistry fast and hence the product formation and temperature rise in the flow is mixing limited. Both reacting and non-reacting simulations were performed with two turbulence models (Smagorinsky and Vreman) and comparisons are made with the available experimental data. Limiters on species concentrations were used to ensure boundedness on these quantities. The simulations show a close agreement of the velocity profiles and the temperature rise profile to that measured in the experiment. The probability density functions have been computed and reveal significant changes in the mixture fractions at the probe locations, showing the effect of heat release on the flow field.