Abstract Phenacyl chloride (1a) reacts with the 3-methylbenzothiazolium quaternary salt (2) to give 4-methyl-2-benzoyl-4H-1, 4-benzothiazine (4a). A mechanism is discussed. Secondary α-haloketones give benzothiazolines. Merocyanine dyes are formed by the reaction of 2 with carbanions. It is shown that 2 functions as a hydride ion abstractor in the formation of merocyanine dyes.