Résumé : A series of novel 1,2,4-pyridothiadiazine 1,1-dioxides (1), some of them being pyridyl analogues of the 1,2,4-benzothiadiazine 1,1-dioxide diazoxide (2), were synthesized and selected physicochemical data (pKa, log P′) were collected. By means of spectral (13C NMR, UV) and X-ray data, the most favourable position of the C=N double bond in the thiadiazine ring was discussed. It was concluded that like 1,2,4-benzothiadiazine 1,1-dioxides, 1,2,4-pyridothiadiazine 1,1-dioxides free of an alkyl substituent in the 2- and 4-positions, whatever the nitrogen atom position in the pyridine ring, show predominance of the tautomeric 4H-form. © 1995.