The design of co-crystals requires knowledge of robust supramol. synthons. The sulfoxide is a potent H bond acceptor and was used as a co-crystal former with a range of NH functional groups, via N-H···O=S H bonds. The NH functional group retains favorable H bond motifs from its own structure in all cases where this is possible, with the sulfoxide interacting in a discrete, capping fashion in four cases and in a bifurcated, bridging fashion in the three other cases presented here. Crystallog. data are given for 7 co-formers, dibenzyl sulfoxide and cyclohexanecarbothioamide.