Methylmethioninesulfonium compounds of the formula ##STR1## wherein X.crclbar. represents an anion;R.sup.1 represents a hydrogen atom or an acyl group of the formula -COR.sup.3 in which R.sup.3 represents an alkyl group with 1 to 20 carbon atoms, the group --CH.sub.2).sub.2 COOH or the group ##STR2## in which Z represents a direct bond or a methylene or vinylene linkage, Y represents C or N, R.sup.4 represents a member selected from the class consisting of a hydrogen atom, lower alkyl groups, lower alkoxy groups, di-lower-alkylamino groups and a sulfamoyl group, n is a number of 1 to 3, and two or more R.sup.4 groups may be identical or different; andR.sup.2 represents the group --COOR.sup.5 in which R.sup.5 represents a hydrogen atom, an alkyl group with 1 to 5 carbon atoms or a metal- or metal complex-forming moiety, or the group ##STR3## in which R.sup.6 groups may be identical or different, and each represent a hydrogen atom or an alkyl group with 1 to 5 carbon atoms, with the proviso that when R.sup.1 represents a hydrogen atom, R.sup.2 is not --COOH; when R.sup.1 represents a hydrogen atom and R.sup.2 represents --CONH.sub.2, X.crclbar. is not Cl.crclbar.; and when R.sup.1 is --COCH.sub.3 and R.sup.2 is --COOCH.sub.3, X.crclbar. is not I.crclbar..The compounds can be easily prepared from methionine as a starting material by a combination of known unit reactions such as methylsulfonium-forming reaction, acylation, amidation, esterification and metal salt-forming reaction. These compounds are especially useful for treating ulcers.