The mechanism of alkylation in the synthesis of zotepine, a commercial anti-psychotic drug, was studied. The alkylation of 2-chloro-10,11-dihydro-dibenzo [b,f]thiepin-11-one (CT) was carried out in a solvent containing potassium carbonate as a base. Examination of the reaction field and reaction rate suggested that the deprotonation and succeeding alkylation of CT took place on the surface of potassium carbonate, and the rate-control ling step for the reaction was determined to be the deprotonation of CT on the surface of potassium carbonate. In addition, the overall reaction rate increased as the particle size of potassium carbonate decreased. Measurements of particle size distributions of potassium carbonate revealed that the addition of water to the solution was effective for fracturing potassium carbonate particles of 250 to 420 mum and thereby enhancing the reaction rate. The overall reaction progress was well expressed by taking account of the increase in the surface area of potassium carbonate due to fracturing in the expression for the overall reaction rate.