The invention relates to a novel process for preparation of N-[2-n-butyl-3-[4-[3-(di-n-butylamino)-propoxy]-benzoyl]-benzofuran-5-yl]methanesulfonamide (I) and pharmaceutical acceptable salts thereof, where a salt of (5-amino-2-butyl-1-benzofuran-3-yl){4-[3-(di-n-butylamino)propoxy]phenyl}methanone of formula (II)—where A is a mono- or dibasic acid forming an acid addition salt with the compound of formula (II), n is 1 if A is dibasic acid and n is 1 or 2 if A is a monobasic acid—is reacted with a mesylating reagent in a heterogen reaction, if desired, in the presence of a phase transfer catalyst. The invention also relates to the novel salts of compound of formula (II), for the preparation thereof and their use in the preparation of dronedarone.