Preoccupancy purging of air-conditioned rooms is practiced in buildings in order to remove air contaminants so that acceptable indoor air quality can exist at the start of occupancy, in accordance to standards such as ASHRAE Standard 62-2001. During a typical daily exercise, the fresh intake and exhaust air dampers are fully opened to allow adequate ventilation with the outdoors. In the present work the effectiveness of preoccupancy purging in an office building of a university in a tropical climate is studied. Observations of indoor CO2 levels, relative humidity and temperature around the time of preoccupancy purging are made. The duration of purging is varied so as to assess the optimum period. Preliminary results indicate that that preoccupancy purging may not be required for the building of study. It is also found that the current practice of preoccupancy purging promotes the growth of mold. © 2011 Published by Elsevier Ltd.