This paper examines thermal mixing of outdoor and return air streams in typical air-handling units equipped with parallel blade dampers. The mixing of the two air streams in rectangular and square mixing chambers is studied for eight different dampers and blade orientations. Testing is conducted at a total supply air of 16000 CFM with outdoor flow percentages of 15% and 30%. The temperature differential between the outdoor and return airflows is kept at 40°F Damper blades with chords of 4 and 6 inches are tested. To examine the mixing of the two air streams, temperature distributions in the mixed region are measured by an array of thermocouples and utilized to compute the thermal mixing effectiveness of the mixed flow. It appears that the relative flow momentum of the two streams plays an important role in the mixing Values of range mixing effectiveness increase significantly with increase in the return air velocity However, this conclusion is not applicable to configurations with minimum outside air in which cold spots can occur downstream in the mixing box. Retesting of configurations producing the least stratification at 50% total supply airflow shows that thermal mixing is degraded, particularly for the cases of 15% outside air.