MP2(fc)/6-31G** calculations on the ground and transition state structures for intramolecular rearrangements of formylcyclopropene found that circumambulation of the ring by a formyl group occurs through a high-energy (DE = 42.6 kcal mol Γ1 ) intermediate with a structure similar to bicyclo[1.1.0]oxide zwitterion. By passing through two successive transition state structures this intermediate rearranges also to topomers of monoepoxide of cyclobutadiene (Dewar furan). Another intramolecular rearrangement of formylcyclopropene leading to inversion of stereochemical configuration at the sp 3 -carbon centre is associated with a one-step transposition reaction which involves a CC bond scission. The energy barrier to the circumambulatory rearrangement of formylcyclopropene can be considerably decreased by protonation of the carbonyl group resulting in formation of the homoaromatic 4-hydroxycyclobutenyl cation. Exo and endo conformers of this cation are susceptible to lowenergy barrier (10.1 and 10.3 kcal mol Γ1 , correspondingly) circumambulatory rearrangement due to migration of a hydroxymethylene group along the periphery of the three-membered ring. The rearrangement occurs as a nonconcerted two-step reaction involving intermediacy of bisected exo and endo conformers of hydroxycyclopropenylcarbinyl cation, and results in inversion of stereochemical configuration at the migrating carbon centre. Homoaromaticity of 4-hydroxycyclobutenyl cation is manifested by the predicted high energy barrier (13.1 kcal mol Γ1 ) against its planar inversion.