In the crystal structures of the title compounds, C(17)H(16)N(2)O(4)S, (I), and C(12)H(14)N(2)O(4)S, (II), the co-existence of both possible keto/enol (4-keto and 4-hydroxy) tautomers, with visible predominance of the 4-keto form, is observed. The tautomeric equilibrium is stabilized by strong intramolecular O-H...O hydrogen bonding. The (13)C NMR spectra recorded for (I) and (II), and theoretical calculations at the RHF SCF ab initio 6-31G** level, show the same tautomeric equilibrium in solution and the gaseous phase. The partially saturated thiazine rings in the pyrido[3,2-e][1,2]thiazine rings systems of (I) and (II) adopt a diplanar conformation. The molecular packing in (I) is influenced by weak intermolecular C-H...O hydrogen bonding and C-H...pi interactions. In the crystal structure of (II), the molecules are linked by a combination of O-H...O hydrogen bonding and C-H...pi and pi-pi interactions.