The presence of radioactive gases in a plume, such as could occur in the event of a nuclear reactor accident, leads to the addition of buoyancy to the plume at an approximately linear rate. Buoyancy might also be added to plumes by chemical reactions, radiation, or latent heat exchange. The resulting plume rise is calculated by a model similar to that used to calculate the rise of ordinary plumes. The case of a radioactive plume is examined in some detail. For the slow leakage of radioactive gases assumed to occur under postulated reactor accident conditions, plume rise due to radioactivity can be neglected. For more rapid leakage rates or for larger reactors, plume radioactivity leads to significant rises.