Crystal structure, 1H and 13C nmr spectroscopy and pKa measurements of 1,2,6-thiadiazin-3,5(2H,6H)-dione derivatives related with phenylbutazone are reported. An anomalous variation in the pKa values of compounds 1,2, and 3 has been observed. Antiinflammatory, analgesic, and antipyretic activities of these compounds have been evaluated.