This contribution describes the study of hydrogen bonding in secondary oxamides, monothiooxamides, and dithiooxamides by ab initio calculations, X-ray diffractions, NMR spectra, thermal analysis, and variabletemperature infrared and Raman spectroscopy. The results can all be interpreted as a function of the change in the strength and the nature of the hydrogen bonding by substituting oxygen for sulfur in the series CH 3 -HNCOCONHCH 3 , CH 3 HNCSCONHCH 3 , CH 3 HNCSCSNHCH 3 and by changing the steric influence of the alkyl group in a series of oxamides (RHNCOCNHR; R ) CH 3 , C 2 H 5 , iC 3 H 7 , tC 4 H 9 ).