Through field studies in large commercial buildings and reviews of building plans, we investigated the effective leakage areas (ELAs), air-leakage rates, and conduction heat gains of duct systems. Different methods for measuring air-leakage rates were also compared. ELAs of supply ducts ranged from 0.4 to 2.0 cm2 per square meter of floor area served, and from 1.0 to 4.8 cm2 per square meter of duct surface area. On a per-unit-floor-area basis, these duct ELAs are comparable to the values measured in residences. The corresponding values of duct leakage class were 60 to 270, much higher than the range of 3 to 12 reported by ASHRAE [ASHRAE Standard 111-1988, Practices for Measurement, Testing, Adjusting, and Balancing of Building Heating, Ventilation, Air Conditioning, and Refrigeration Systems, American Society of Heating, Refrigerating, and Air Conditioning Engineers, Atlanta, 1988] as attainable for quality duct construction and sealing practices when leakage at connections to duct-mounted equipment is not considered. The measured air-leakage rates as a percentage of the inlet air flow rate varied from 0% to 30%, with most of the measurements falling between 10% and 20%. Large inconsistencies among the air-leakage rates determined from different measurement procedures exemplify the need for further development and evaluation of measurement methods. Heat gains between the outlet of the cooling coils and the supply registers caused supply air temperatures to increase, on average, by 0.6°C to 2°C. The corresponding values of conduction effectiveness were 0.75 to 0.90; thus, heat conduction decreased the cooling capacity of the supply air exiting registers by 10% to 25%. Because these results are based on studies in only a few buildings, generalizations from these findings are premature.