The standard thermal treatments proposed by Lima-de-Faria have been applied to metamict euxenites, polymignites, yttrotantalites, samarskites, pyrochlores, and allanites. In the case of these metamict niobotantalates a facecentred cubic phase is formed at 700° C., and at 1000°C. a more characteristic phase appears combined with the cubic one. In the case of allanites only a slight formation or strengthening of the original structure of allanite appears at 700°C., and at 1000°C. a cubic phase is formed together with other phases.