This paper presents findings from a recent call centre study conducted in an air-conditionedoffice in the Tropics. The effects of temperature and outdoor air supply rate, each kept at twoset points within currently acceptable limits, were explored in a 2 2 balanced experimentaldesign over a 9-week period. The results indicate that both a slightly cooler thermalenvironment and improved indoor air quality improve performance by reducing average callduration. Temperature and outdoor air supply rate were synergistically related to one anotherin affecting operator performance (P < 0.062). Performance improved by 4.9% (P < 0.05)when the air temperature was decreased from 24.5 to 22.5C at the lower outdoor air supplyrate (9.8 l/s/p) and improved by 8.8% (P < 0.04) when the outdoor air supply rate wasincreased to 22.7 l/s/p at the higher air temperature. These outdoor air supply rates are basedon actual occupancy.