AbstractIt is well known that the 5,5‐disubstituted barbiturates adopt the planar conformation in solution. However, kinetic investigations on the hydrolysis of spiroalkane‐1′,5‐barbiturates and the analysis of the C=O stretching vibrations showed the fivefold increase of the hydrolysis rate constants and decrease of the conjugation in the pyrimidine ring for the spirocyclobutane‐1′,5‐barbituric acid (2) in relation to those containing a cyclopentane or a cyclohexane ring. To explain these phenomena we postulated the twisting of one of the imide bonds due to the steric strain of the cyclobutane ring. Very recently we reported on our investigations on the hydrolysis of spirocyclopropane‐1′,5‐barbituric acid (1) and the13C‐NMR data of four spirobarbiturates. We concluded, that the conjugation between the cyclopropane ring and the C‐4 or C‐6 carbonyl group, possible due to the non‐planar conformation of the barbiturate ring, is responsible for the low hydrolytic reactivity of the pyrimid