The two diastereoisomeric 1,2-epoxy-1-phenyl-4-t-butylcyclohexanes and the corresponding four diols are converted into mixtures of 2-phenyl-5-t-butylcyclohexanones and 1-phenyl-3-t-butylcyclopentanecarbaldehydes by boron trifluoride–ether complex. The steric course of these reactions is interpreted in terms of a mechanism involving open carbonium ion intermediates for the diols, whereas the rearrangement of the epoxides appears to be more concerted. The silver oxide-promoted rearrangements of the cis-chlorohydrins of the same series seem to be entirely concerted.