Singapore 5-Cyclohexylmethylenehydantoin and its 1-and 3-methyl derivatives were studied as the aliphatic analogues of the 5benzylidenehydantoins to examine the effects of replacing a benzene by a cyclohexane ring on Z-E isomerism. The relative stabilities of the Zand E-isomers were compared by estimating the free energy differences from thermal equilibration experiments and the heats of formation by AM1 calculation. I n the Z-isomers, the possible existence of weakly attractive interaction between the C(4) = 0 and C(6b-H groups, which form parts of a stereochemically favourable, conjugated a,B-unsaturated system, is suggested as a contributory factor to the observed configurational preference. Assignment of configurations was based mainly on N M R and is supported by IR and UV spectra. N M R spectra also revealed the conformation of the cyclohexane ring and the cyclohexyl proton signals were assigned by double resonance and by high-field N M R in two solvents where solubility allows.