Measurement of air exchange rates, ages of air, and nominal and local ventilation efficiencies in large buildings is often complicated by the building size and compartmentalization, and by the presence of multiple ventilation systems. To allow characterization of the ventilation process in such buildings, a unique experimental system, that employs multiple tracer gases, is being developed at Lawrence Berkeley Laboratory. The tracer gases are sulfur hexafluoride and five halocarbons. The system is designed to be non-obtrusive, highly automated, and relatively easy to ins tall in buildings. Included in the system is a programmable tracer-gas injector thac automatically initiates and terminates the process of tracer injection. One injector will be used for each tracer gas. Another component of the system is a programmable sampler that collects up to 15 small samples of air; these samples can be stored and analyzed at a later time in the laboratory. One sampler will be placed at each sampling location. Because each tracer gas injector and sampler is a stand-alone device, long runs of tubing are not required to inject tracer or take samples. The concentrations of tracer gas in the samples are determined using a gas chromatograph with an electron capture detector. This paper first provides background information on the ventilation of large buildings and a discussion of age of air and ventilation efficiency. Various tracer gas techniques are briefly reviewed and equations to analyze the tracer gas data from the step-up and decay techniques are presented. The experimental system is described in detail and the results of two tests of system performance are presented. In these tests, four of the six tracer gases yielded the same result (i.e. air exchange rate, age of air, or ventilation efficiency) within approximately 15 percent. In an experiment conducted in a well-mixed test space, these four tracer gases also yielded an air exchange rate, or age of air, that differed by no more than 12 percent from a reference measurement made with an orifice plate flow meter. Further work is required to increase the accuracy of our measurements with the other two tracer gases.