Benzothiazocine and benzothiazonine derivatives of formula (I): ##STR1## in which: R.sup.1, R.sup.2 and R.sup.3 are the same or different and each represents a hydrogen atom or an alkyl group;X.sup.1 and X.sup.2 are the same or different and each represents a hydrogen atom, an alkyl group, an alkoxy group, an alkanesulphinyl group, an alkanesulphonyl group, a halogen atom, a trifluoromethyl group, a nitro group or a cyano group;n is 1 or 2; andm is 0 or 1 (excluding those compounds wherein:R.sup.1, R.sup.2, R.sup.3, X.sup.1 and X.sup.2 all represent hydrogen atoms;n is 1 and m is 0; and wherein:R.sup.1 and R.sup.2 both represent hydrogen atoms;R.sup.3 represents a hydrogen atom or a methyl group;X.sup.1 represents a chlorine atom at the 9-position;X.sup.2 represents a hydrogen atom;n is 1; andm is 0)and acid addition salts thereof are novel compounds which have been found to suppress gastric secretions and are thus useful in the treatment and prevention of gastric ulcers. Compounds in which m is 0 may be prepared by reacting a corresponding .omega.-substituted thioalkylaniline derivative with a halogen or an active halogen compound and then condensing the product and, if necessary, introducing an alkyl group at the 1-position. Compounds in which m is 1 may be prepared by oxidizing the corresponding compound wherein m is 0.