A green process for the synthesis of 11-{4-[2-(2-hydroxyethoxy)ethyl]-1- piperazinyl}dibenzo[b,f][1,4]thiazepine by the reaction of 11-(1-piperazinyl) dibenzo[b,f][1,4]thiazepine or its dihydrochloride salt with 2-(2-chloroethoxy) ethanol in the presence of an inorganic base and water is reported (conversion 99.9 % in a short time and without any impurities). The metal halides and phase transfer catalyst increase the rate of reaction, especially in water as the solvent.