An order of magnitude analysis is presented which shows that large-scale mixing in gas turbine combustors controls the stoichiometry of the combustion process at typical engine operating conditions. Fuel lean combustion, a desirable condition for low NO x -formation rates, is thereby precluded for those combustors where fuel and air are fed separately. This is due to the fact that combustion times in the flammable range are very much shorter than mixing time. Low-pressure combustor test data indicate a fixed rate of NO x generation consistent with a mixing rate-limited mechanism. Significant levels of free carbon in lean mixtures support the conclusion.