In 1891 Le Chatelier [6] first proposed an empirical mixing rule for predicting the flammable limit of lean fuel air mixtures - this method is still widely used today. This paper presents a proof to Le Chatelier's mixing rule based on thermodynamics. A number of assumptions are required to produce a proof. These assumptions are The product heat capacities are constant The number of moles of gas is constant. The combustion kinetics of the pure species are independent and unchanged by the presence of other combustible species. The adiabatic temperature rise at the flammability is the same for all species. These assumptions were found to be reasonably valid at the lower flammability limit and less so at the upper flammability limit. This paper also considers experimental data that support some of the assumptions made to arrive at the Le Chatelier mixing equation.