The kinetics of the polycondensation between diphenyl-dichlorosilane and bisphenol-A in equal mole ratio has been studied．As the velocity of the polvcondensation in the molten state is unstable，the kinetic order can not be evaluated．However，after dilution with solvents such as naphthalene，diphenyl ether，acetophenone，etc．to certain degree，the kinetics of the polycondensation in the initial stage followed a second order reaction． Fur- thermore，at the same temperature，the stronger is the polarity of the solvent，the greater is the reaction rate．By plotting the logarithm of rate constants in certain range of tempera-tures against the reciprocal of the absolute temperature．a linear relationship is obtained．The activation energy of the polycondensation in naphthalene，diphenyl ether， and aceto-phenone was found to be 17.1．24.8 and 10.3 kcal/mole respectively．The reaction may be catalysed bv the Lewis bases and their salts．It is assumed that the complexity of the polycondensation kinetics in the molten state is a manifestation of the changing reaction rate due to the changing polarity of the polycondensation system． An SN2 mechanism for the reaction has been suggested．