Silica-alumina catalysts of varying acidity were used for polymerization of hexamethyl cyclotrisiloxane and octamethyl cyclotetrasiloxane monomers (cyclic trimer and tetramer, respectively). Acidic sites of silica-alumina are responsible for the polymerization and Variation in the acidity of the catalyst were shown to influence the polymerization significantly. Mordenite type zeolites with low silica-to-alumina ratio (similar to 5) gave low yields of polymer from tetramer and zeolite ZSM-5 with a much higher silica-to-alumina ratio (similar to 40) was found to be a very efficient catalyst for polymerization. Studies on the effect of water on the polymerization suggested that the polymerization (rate and yield) depended to some extent on the amount of water present in the system. The products were characterized using FTIR, H-1-NMR, Si-29-NMR, and GPC techniques. A plausible mechanism for polymerization of cyclic siloxane monomers on silica-alumina catalysts was proposed.