For in-pile sweep-gas experiments it is sometimes necessary to deduce the time-dependent release rate of a radioactive gas from a specimen by measurements made at some point downstream. An experimental method to measure the amount of dispersion of the radioactive gas in the sweep gas is described. By this method, correction factors can be applied to a measured wave shape to obtain the generated wave shape. A theoretical model, assuming axial turbulent flow of the sweep gas with a flat radial distribution, has been developed. Application of the model involves an experimentally determined parameter “a” for the particular sweep-gas system used. The agreement between the experiment and the theoretical model is excellent.