A number of acid salts of simple dicarboxylic acids crystallize with their molecules linked into infinite chains by “very short” hydrogen bonds. In this type of H-bond two oxygen atoms, belonging to adjacent molecules are related by a centre of symmetry or by other symmetry element, and the proton lies on the symmetry elemept. These structures have been referred to as Type A2structures and present a characteristic i.r. spectrum of Hadzi's type1. Examples of salts showing this behaviour are given by the potassium hydrogen ma-lonate (KHM)2, potassium hydrogen succinate (KHS)3and potassium hydrogen glutarate (KHG)4, whose crystal structures have been studied by J.C. Speakman and his coworkers.