A process for the ring-opening polymerisation of cyclosiloxanes, which comprises contacting a cyclosiloxane with 1 - 500ppm by weight of a phosphazene base, based on the weight of cyclosiloxane, in the presence of water, preferably 0.5 - 10 mols per mol of phosphazene base. Also claimed is a process in which the cycloxilosane is present together with an agent which inhibits catalyst activity of the phoshpazene base, e.g. carbon dioxide or water, and in which the polymerisation is initiated by reducing the effect of the inhibiting agent, e.g. by heating.