Abstract: A preparation process of high purity bisphenol A from phenol and acetone with a continuous or batch type reactor by on-line estimating with a computer the composition of the reaction mixture and saturated dissolving temperature of bisphenol A under the reaction conditions and by automatically controlling the reaction conditions and liquid transfer conditions of the reaction mixture to the next step. The process does not form crystals of bisphenol A-phenol adduct in the reactor and cooler. A maximum rate of reaction can be obtained. Consequently, fluctuation of reaction temperature can be prevented and the amount of impurities formed is small. The discharge operation of the reaction mixture from the reactor can be carried out steadily and with ease, and does not disturb the operation of successive steps.