The title solvates, (I) and (II), both C(18)H(16)N(6)O(3).C(2)H(6)OS, are isomeric. The conformations adopted by the 6-substituent are significantly different, with the 6-aminophenyl unit remote from the nitrophenyl ring in methoxypyrimidine (I) but adjacent to it in pyrimidinone (II). Pairs of pyrimidine molecules in (I) are linked by N-H...N hydrogen bonds to form cyclic centrosymmetric dimers from which the dimethyl sulfoxide molecules are pendent, while in (II) a combination of three independent N-H...O hydrogen bonds links the components into a chain containing both R(2)(2)(8) and R(4)(2)(8) rings, in which the dimethyl sulfoxide component acts as a double acceptor of hydrogen bonds. The significance of this study lies in its observation of different conformations for the pyrimidine components in (I) and (II), and different hydrogen-bonded structures, apparently dominated by the different roles adopted by the dimethyl sulfoxide components.