At the present time, cerium-144 and premtheium-147 are the two fission product rare earths that appear most promising for use as the heat source in isotopic power units. Under proper conditions, cerium and the trivalent rare earths can be extracted from the Purex fission product waste stream as an insoluble sodium-rare earth double sulfate. A reprecipitation as the double sulfate, dissolution of the hydroxide, serves to give almost complete separation from the corrosion products, inert constituents of the waste, and from most of the fission products. The cerium and the trivalent rare earths must then be separated from each other. In the case of cerium recovery, it is necessary to remove the trivalent rare earths in order to maximize the specific activity of the cerium. If promethium is the desired product, a preliminary cerium separation is desirable to protect the ion-exchange resin (used for separating promethium from its adjacent rare earths) from the intense high-energy radiation from cerium.