Process for producing bisphenol A

A process for producing bisphenol A, which comprises:

• reacting phenol with acetone in the presence of an acid ion-exchange resin as a catalyst and a cocatalyst consisting essentially of an alkyl mercaptan, wherein a concentration of an organosulfur compound in a crystallization starting material obtained by conducting the reaction under conditions that a reaction temperature is between 60 and 100° C., a phenol to acetone molar ratio is between 6 and 13 and an acetone to alkyl mercaptan molar ratio is between 13 and 25, distilling off unreacted acetone, by-product water and the alkyl mercaptan as the cocatalyst and further distilling off excess phenol is maintained at 200 weight ppm or less;
• separating the crystallization starting material into phenol adduct crystals and a crystallization mother liquor;
• and
• recycling the crystallization mother liquor, wherein a part or the whole of the crystallization mother liquor is isomerized and recycled into the crystallization starting material.