Improper mixing of outside air and return air streams in building air conditioning systems has been recognized for years. The problems may lead to nuisance cycling, frequent freeze-stat trips and serious consequences of a frozen or ruptured conditioning coil. It was thought that typical solutions for the problem usually consist of preferred placement of outside air and return air duct penetrations to the mixing box, manipulation of the inlet damper angles and velocity ratio between the outside air and return air streams and the insertion of static flow mixers in the mixing box to help improve the thermal stratification. This paper reports the results of a computational fluid dynamics (CFD) study conducted as a follow-up to an experimental study conducted at the Ruskin Laboratory in Grandview, Missouri, sponsored by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). The CFD results indicated that the most significant improvement in mixing performance with minimum increase in pressure drop and energy use is achieved by expanding the mixing plenum. Effectiveness increased from 39 percent to 67 percent with less than a 0.1 inch of water additional pressure drop. However, optimization of relative plenum dimensions and baffle size and placement awaits additional CFD simulations and full scale validation.