The fulminates form a class of compounds isoelectronic with the azides and cyanamides. They are, however, extremely sensitive materials and even the ionic salts explode. A study has been made of some of the optical properties of small crystals and thin deposited films, and of the solid state infrared spectra, in order to determine the nature of the bonds in the sodium, potassium, thallous, silver and mercury salts. Structural data is also available and it is concluded that while sodium, potassium and thallous fulminates are ionic solids, the mercury salt is covalent. It is likely that in the silver salt there is also a high degree of directed bonding in the solid state. The nature of the bond between the metal and the carbon atom of the fulminate group can, however, be different in the silver and mercury salts and this will be reflected in the order of the stability of the salts.