The BF₃:etherate catalysed reactions of a series of ethylene oxides contained in a 4-tert-butylcyclohexane system have been examined. Rearrangements of the epoxides to give carbonyl products have been explained in terms of discrete carbonium ion intermediates. The 4-tert-butylcyclohexanes (7,8) give the isomeric aldehydes (9,10) which must arise from [diagram] species with considerable rotational ability. Attack by fluoride to give fluorohydrins (11,12), and by aldehyde (7,8) at epoxide to give dioxolanes (14,15,16,17) has been shown to occur exclusively by reaction at the tertiary centre. The rearrangements of the 1-methyl (45,46), 2-methyl (47,48) and 1,2-dimethyl. (75,76) 4-tert-butylcyclohexane oxides have been rationalised in terms of tertiary carbonium ion intermediates. The initial conformation and structure of the carbonium ion is determined by the requirement that maximum residual overlap of the departing-OBF₃ and the developing electron deficient centre occurs in the transition state. The monomethyl epoxides (45,48) give a carbonium ion initially in a boat conformation which undergoes conversion to a chair form at a rate comparable to migrational processes. The epoxides (46,47,75,76) are considered to give directly the chair conformation of the carbonium ion intermediate. Significant fluorohydrin formation is observed from all epoxides. The structure of these products is rationalised in terms of a preference for axial addition by the fluoride species and appreciable charge development at the tertiary centre.