Treatment of (3,4-dihydro-4-hydroxy-3-oxo-2H-1,4-benzothiazin-2-yl)acetic acid (1a) with sodium hydroxide yields the corresponding lactam,i.e. (3,4-dihydro-3-oxo-2H-1,4-benzothiazin-2-yl)acetic acid, together with the α,β-unsaturated acid, 3,4-dihydro-3-oxo-2H-1,4-benzothiazine-Δ2,α-acetic acid. The 6-methyl- and 6-bromo-derivatives of1abehaved similarly when reacted with sodium hydroxide but when 3,4-dihydro-4-hydroxy-3-oxo-2H-1,4-benzothiazine was so treated a more complex reaction occurred.Methyl (6-bromo-3,4-dihydro-4-hydroxy-3-oxo-2H-1,4-benzothiazin-2-yl)acetate was also treated with hydrochloric acid. The two products isolated were (6-bromo-3,4-dihydro-3-oxo-2H-1,4-benzothiazin-2-yl)acetic acid and (6-bromo-7-chloro-3,4-dihydro-3-oxo-2H-1,4-benzothiazin-2-yl)acetic acid.The action of hydrochloric acid on 3,4-dihydro-4-hydroxy-7-methyl-3-oxo-2H-1,4-benzothiazine also gave two products. One was the corresponding lactam; the other was unexpected and has been tentatively identified as bis[2-(3,4-dihydro-7-methyl-3-oxo-2H-1,4-benzothiazine].