Both title structures exhibit essentially planar barbiturate rings. The crystal structure of enallylpropymal (5-allyl-5-isopropyl-1-methylbarbituric acid), C(11)H(16)N(2)O(3), is composed of centrosymmetric N-H...O hydrogen-bonded dimers, while 1,5-di(but-2-enyl)-5-ethylbarbituric acid, C(14)H(20)N(2)O(3), forms N-H...O hydrogen-bonded helical chains. Each of the ten known crystal structures of closely related N-monosubstituted derivatives of barbituric acid displays one of the fundamental N-H...O hydrogen-bonded motifs of the two title structures, i.e. either a dimer or a chain.