A number of 7H-pyrimido[4, 5-b]thiazines are obtained by reacting 5-amino-6-mercaptopyrimidines with α-halogeno ketones. It is established that the reaction takes place in three stages: formation of 5-amino-6-[β-ketoalkyl(aralkyl)-mercapto]pyrimidines, cyclization of these to 6-hydroxy-5, 6-dihydropyrimido[4, 5-b][1, 4]thiazines, and dehydration of the latter to 7H-pyrimido[4, 5-b][1, 4]thiazines.