Abstract: Antimicrobial compositions comprising as active ingredient at least one compound of the general formula ##STR1## (wherein R.sup.1 and R.sup.5, which may be the same or different each represent a hydrogen or halogen atom and R.sup.2, R.sup.3 and R.sup.4 which may be the same or different, each represent a hydrogen atom or a lower alkyl group) in association with an antimicrobial carrier, excipient or diluent. The compounds of formula I may be prepared by reacting a corresponding dihydrothiazolo 8-oxide compound with a base. Certain of the compounds of formula I are novel and may also be prepared by pyrolytic decarboxylation of a corresponding carboxyl substituted thiazolo-8-oxide compound.