Processes for preparing sulfoxides useful as drugs such as acid secretion inhibitors or antiulcer drugs or intermediates for the preparation of drugs in high yields, at high purities, and with safety. Specifically, a process for the preparation of sulfoxides (II) by oxidizing a thio ether (I) with a peroxoborate salt in the presence of an acid anhydride or a metal catalyst; and a process for the preparation of sulfoxides (II) by oxidizing a thio ether (I) with an N-halosuccinimide, 1,3-dihalo-5,5-dimethyl-hydantoin or dichloroisocyanuric acid salt in the presence of a base. In said formulae R1 is hydrogen, methoxy or difluoromethoxy; R2 is methyl or methoxy; R3 is 3-methoxypropoxy, methoxy or 2,2,2-trifluoroethoxy; and R4 is hydrogen or methyl