The understanding of intermolecular interactions is a key objective of crystal engineering in order to exploit the derived knowledge for the rational design of new molecular solids with tailored physical and chemical properties. The tools and theories of crystal engineering are indispensable for the rational design of (pharmaceutical) cocrystals. The results of cocrystallization experiments of the antithyroid drug 6-propyl-2-thiouracil (PTU) with 2,4-diaminopyrimidine (DAPY), and of 6-methoxymethyl-2-thiouracil (MOMTU) with DAPY and 2,4,6-triaminopyrimidine (TAPY), respectively, are reported. PTU and MOMTU show a high structural similarity and differ only in the replacement of a methylene group (–CH2–) with an O atom in the side chain, thus introducing an additional hydrogen-bond acceptor in MOMTU. Both molecules contain anADAhydrogen-bonding site (A= acceptor andD= donor), while the coformers DAPY and TAPY both show complementaryDADsites and therefore should be capable of forming a mixedADA/DADsynthon with each other,i.e. N—H...O, N—H...N and N—H...S hydrogen bonds. The experiments yielded one solvated cocrystal salt of PTU with DAPY, four different solvates of MOMTU, one ionic cocrystal of MOMTU with DAPY and one cocrystal salt of MOMTU with TAPY, namely 2,4-diaminopyrimidinium 6-propyl-2-thiouracilate–2,4-diaminopyrimidine–N,N-dimethylacetamide–water (1/1/1/1) (the systematic name for 6-propyl-2-thiouracilate is 6-oxo-4-propyl-2-sulfanylidene-1,2,3,6-tetrahydropyrimidin-1-ide), C4H7N4+·C7H9N2OS−·C4H6N4·C4H9NO·H2O, (I), 6-methoxymethyl-2-thiouracil–N,N-dimethylformamide (1/1), C6H8N2O2S·C3H7NO, (II), 6-methoxymethyl-2-thiouracil–N,N-dimethylacetamide (1/1), C6H8N2O2S·C4H9NO, (III), 6-methoxymethyl-2-thiouracil–dimethyl sulfoxide (1/1), C6H8N2O2S·C2H6OS, (IV), 6-methoxymethyl-2-thiouracil–1-methylpyrrolidin-2-one (1/1), C6H8N2O2S·C5H9NO, (V), 2,4-diaminopyrimidinium 6-methoxymethyl-2-thiouracilate (the systematic name for 6-methoxymethyl-2-thiouracilate is 4-methoxymethyl-6-oxo-2-sulfanylidene-1,2,3,6-tetrahydropyrimidin-1-ide), C4H7N4+·C6H7N2O2S−, (VI), and 2,4,6-triaminopyrimidinium 6-methoxymethyl-2-thiouracilate–6-methoxymethyl-2-thiouracil (1/1), C4H8N5+·C6H7N2O2S−·C6H8N2O2S, (VII). Whereas in (I) only anAA/DDhydrogen-bonding interaction was formed, the structures of (VI) and (VII) both display the desiredADA/DADsynthon. Conformational studies on the side chains of PTU and MOMTU also revealed a significant deviation for cocrystals (VI) and (VII), leading to the desired enhancement of the hydrogen-bond pattern within the crystal.