On the assumption that the velocity and density distributions across a jet of one gas issuing into a stream of a second gas are of exponential form, the momentum integral approach has been employed to find the variation of centre-line velocity downstream of the potential core. The results for the particular case of a jet issuing into a gas at rest are equivalent to those found theoretically by Blottner (Ref. 2) and experimentally by Keagy and Weller (Ref. 1). The results for the jet issuing into a moving stream agree with the limited experiments of the present author. These calculations show the importance of jet to free-stream density ratio with respect to the rate of decay of the jet downstream of the mixing region.