Investigations were made of the chemical behaviour and ultra-violet absorption spectra of the crystalline tetramer of hydrogen cyanide (and derivatives) in an attempt to establish whether the tetramer normally exists as aminoiminosuccinonitrile or diaminomaleinitrile. Chemical studies including partial hydrolysis, ozonolysis, attempted replacement of a possible labile hydrogen and condensation of a second aldehyde molecule were all inconclusive. Although other workers have established the existence of aminoiminosuccinonitrile, the spectroscopic measurements indicate that the tetramer normally exists as diaminomaleinitrile. These apparently conflicting results may be explained by the tautomeric relationship between the two forms, which intern explains the inconclusive chemical results which have arisen in so many investigations of the tetramer.