The influence of rare earth compounds on the polymerization of cyclosiloxanes has been studied for hexamethylcyclotrisiloxane (D-3) and octamethylcyclotetrasiloxane (D-4). In both cases, it has been shown that rare earth compounds alone do not lead to well defined polymers : yields are never quantitative and the formation of cyclic side-products is always observed. The influence of these compounds has also been studied in the anionic polymerization of the same cyclosiloxanes initiated by organolithium reagents or fluorides. In the case of D-3, it has been shown that back-biting reactions can be slowed down in the presence of rare earth compounds whereas for D-4, both polymerization and back-biting reactions get slower.