alpha,omega-Bis(4-hydroxybutyl) polydimethylsiloxane (molecular weight 1500 to 4500) can be prepared with well-controlled molecular weight through the reaction of 1,3-bis(4-hydroxybutyl)tetramethyl disiloxane and dimethoxydimethyl silane or diethoxydimethyl silane in the presence of a stoichiometric amount of water and hydrochloric acid. The molecular weight of these hydroxybutyl-terminated polysiloxanes can be determined fairly consistently by a titration method. These polysiloxanes are stable toward cyclization.