Incomplete mixing in ventilation ducts may cause problems when measuring air temperatures. This is particularly true for mixing boxes in air-handling units. Examples of problems are; unit shutdowns due to low limit thermostats, frozen Goils, non-optimal　 control, increased energy use and unsuccessful fault detection. This paper presents measurements from an air-handling unit mixing box in an office building. The mixing of air in the mixing box is documented under various outside air temperatures and damper positions. Comparisons between standard single-point sensors and averaging sensors are resented .The main results are that the temperature difference between the lower and the upper part of the duct downstream the mixing box can be considerable and increases as outside air temperature decreases. Standard single-point temperature sensors are not sufficient when measuring temperature in mixing boxes.