A simple model is developed to determine the entrainment coefficient and the spread of a light-gas plume in a quiescent atmosphere. Experiments performed with low-velocity saltwater/freshwater and helium-in-air jets indicate that buoyant gas plumes spread significantly faster than thermal plumes. The calculated effluent concentrations are in excellent agreement with those measured when art entrainment coefficient of 0.15 is used in the plume equations. This is significantly higher than the entrainment coefficients of 0.075 to 0.093 reported for thermal plumes.