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).