The anionic emulsion polymerization of octamethylcyclo-tetrasiloxane (D4) has been studied in the presence of dodecylbenzensulfonic acid (DBSA) and its sodium salt. (Na-DBSA) The effects of temperature and amounts of emulsifeir and catylyst on the reaction-rate and M.W. of the polymer were discussed. It is assumed that the anionic emulsion polymerization contains three kinds of elementary reaction: the opening reaction of D4

the addition reaction of D4 to HO-ended siloxane and the condensation reaction between HO-ended siloxane and that the surface of emulsion particles in the main site of the reaction.