The stack effluent of a nuclear fuel reprocessing plant would be expected to contain sufficient tritium to serve as a radioactive tracer for the plume. In order to make use of this built-in tracer, a silica gel sampler for tritiated moisture was developed, which permits large scale sampling. An intensive study of the area surraunding Nuclear Fuel Services, Inc. was undertaken during the summer of 1967 to determine experimentally the maximum concentration (Cmax) of the stack effluent using tritiated moisture as the tracer. Sampling legs that radiated from the stack were established. During a two-month period >700 samples were collected on 7 sampling legs. The average tritium radioactivity on the sampling leg northwest of the plant (leg G) exceeded 1000 tritium units (TU) 71% of the time for the 28 sampling periods studied. In 11 of 28 cases a maximum concentration of >3000 TU occurred. It was definitely demonstrated that a Cmax can be determined by tracing with tritiated moisture. Based on experimental Cmax values, an estimate of the emission rate, Q, was made under various meteorological conditions. The possibility of a secondary saurce of tritiated moisture influencing measurement of stack-emitted tritium was considered.