The molecule of buthalital, (I) [systematic name: 5-(2-methylpropyl)-5-(prop-2-en-1-yl)-2-sulfanylidene-1,3-diazinane-4,6-dione], C11H16N2O2S, exhibits a planar pyrimidine ring, whereas the pyrimidine ring of methitural, (II) [systematic name: 5-(1-methylbutyl)-5-[2-(methylsulfanyl)ethyl]-2-sulfanylidene-1,3-diazinane-4,6-dione], C12H20N2O2S2, is slightly puckered. (I) and (II) contain the same hydrogen-bonded chain structure in which each molecule is connected, via four N—H...O=C hydrogen bonds, to two other molecules, resulting in a hydrogen-bonded chain displaying a sequence of R22(8) rings. The same type of N—H...O=C hydrogen-bonded chain has previously been found in several 5,5-disubstituted derivatives of barbituric acid which are chemically closely related to (I) and (II).