Raman (4000–30 cm−1) and infrared (4000–250 cm−1) spectra of s—diformohydrazide HCONHNHCOH and its N—deuterated compound (HCONDNDCOH) in the crystalline state have been measured at room temperature and at liquid nitrogen temperature. The vibrational assignment of the observed frequencies has been proposed based upon dichroism of single crystals, Raman polarization data, isotopic frequency shifts, and analogies with the spectra of related molecules. The normal coordinate treatment has been made for the in—plane vibrations by using a 21 parameter force field and previous assignments of some infrared bands are also revised.