To compare the structural properties of oxalamide and thiooxalamide groups in the formation of hydrogen bonds suitable for supramolecular assemblies a series of retropeptides was studied. Some of them, having oxalamide bridges, are gelators of organic solvents and water. However, retropeptides with oxygen replaced by the sp(2) sulfur have not exhibited such properties. The crystal structures of the two title compounds are homostructural, i.e. they have similar packing arrangements. The monothio compound crystallizes in the orthorhombic space group P2(1)2(1)2(1) with two molecules in the asymmetric unit arranged in a hydrogen-bond network with an approximate 4(1) axis along the crystallographic b axis. However, the dithio and dioxo analogues crystallize in the tetragonal space group P4(1) with similar packing patterns and hydrogen-bonding systems arranged in agreement with a crystallographic 4(1) axis. Thus, these two analogues are isostructural having closely related hydrogen-bonding patterns in spite of the different size and polarity of oxygen and sulfur which serve as the proton acceptors.