A unique multiple tracer experimental system has been developed and utilized within commercial buildings to monitor ventilation rates, air exchange efficiency, ages of air (at multiple indoor locations), flow rates of supply and outside air, and percent outside air in supply airstreams. The multiple tracer technique also makes it possible to determine the fractions of air at a monitoring point that entered the building through a particular air handler and by infiltration. To label the incoming air, a distinct tracer gas is injected at a constant rate into each outside air or supply airstream. Cart-mounted gas chromatographs are placed in mechanical rooms and monitor tracer gas concentrations versus time in the major airstreams of the air handlers. Small "local samplers" placed at various indoor locations are utilized to monitor local ages of air. Age distribution theory is applied to determine ages of air; however, the standard methods of applying this theory are modified to process the multiple tracer data. The experimental system, methods of data analysis and the results of studies in both a twelve-story building and a complex of three interconnected two-story office buildings are presented. Rates of outside air supply per occupant were comparable to or above the minimum rate of 10 l/s-occupant specified in the draft revised version of the ASHRAE ventilation standard. Within regions of these buildings that are served by a single air handler that supplies a mixture of outside and recirculated indoor air, the measured ages of air varied by 30% or less from the region-average age. Monitoring at different heights above floor level provided no evidence of either a short-circuiting or displacement flow pattern within rooms. The age of air varied more substantially between physically isolated regions of a building, regions served by different air handlers, and over time. In the complex of three buildings, air exchange efficiency values were close to 0.5 suggesting relatively uniform mixing of air in regions served by a single air handler. In the other building, air was supplied and removed from physically- separated regions, and the air exchange efficiency was 0.7.