The design of ventilation performance of air-conditioning systems in large commercial and office buildings is quite established. However, it is not the same with the designs of air-conditioning systems in most residential buildings. Split system air-conditioning units are commonly employed in residential buildings in the tropics due to their convenience in terms of energy conservation, aesthetics, flexibility, acoustic performance and ease of operation. Such units are also popular among small offices, shopping complex and even as supplementary air-conditioning units beyond normal office-hours in large commercial and office buildings. This paper presents findings from a recent study of the ventilation performance and indoor air quality (IAQ) in a master bed room of a condominium unit in Singapore, employed with a split system air-conditioning unit. The attached bathroom is equipped with an exhaust fan, whose operation and its impact on the resulting ventilation characteristics was also studied. Four adults occupied the room throughout the course of the experiments. It was observed that the carbon dioxide level in the bed room can exceed 2000 ppm without the exhaust fan in about 2 h. The operation of the exhaust fan quickly lowered the level of carbon dioxide to about 1000 ppm. The findings suggest the need to design for ventilation provision in split system air-conditioning units. (C) 2004 Elsevier B.V. All rights reserved.