A unified model is used here to determine the distributions of pressure, temperature and velocity that reduce gaseous fuel-air mixing. The model uses the fuel mass fraction within infinitesimal fluid elements and the total derivative of this fraction with respect to time to quantify the degree and rate of mixing, respectively. An Eulerian representation is used. The model is valid for gaseous fuels that contain a single chemical species and for low-pressure combustors. The model permits the presence of only trace amounts of combustion products within the mixing region. Aside from these restrictions, the mixing model can be applied to a variety of combustor designs and operational conditions. Results show that certain distributions of pressure, temperature and velocity at the fuel-air boundary reduce or even prevent gaseous fuel-air mixing, in particular whenever the logarithmic rate of temperature and the velocity divergence are comparable. These conditions yield low intensity and efficiency combustion as well as high pollutants emission level.