Glutaconic acid and its derivatives have for many years been studied by Thorpe and co-workers (J.C.S., 1905, 89, 1669; Thorpe and Thole, J. C. S., 1911, 99, 2187; Goss, Ingold and Thorpe, J. C. S., 1923, 129, 1199; etc.) Compounds of this type, containing a mobile hydrogen atom, exhibit three carbon tautomerism, accompanied by geometrical isomerisom. With the unsubstituted acid, migration of the hydrogen atom may or may not lead to interconversion of the cis and trans forms, and when no interconversion occurs the initial and final forms I and II are indistinguishable.