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For the boron trifluoride catalysedrearrangement of epoxides with the structural feature (I), stereochemical data have been obtained1 which are readily interpreted only in terms of the intermediacy of the discrete carbonium ion (2). As part of a study with the objective of investigating the characteristics of the subsequent rearrangement processes of these carbonium ions (2) we examined the BF,-catalysed rearrangement of the deuterated epoxide (3) (Scheme 1). Comparison of the relative Scheme I yields of isobutyraldehyde, deuterated a t C 1 when formed by hydride shift (4a), and a t C2 by deuteride shift (4b), would allow an estimate to be made of the primary deuterium isotope effect (k,/k,) for the hydride (deuteride) shift. It is necessary to study a n internal HID-competitive reaction since the required information is related to the fast reaction in the two-step rearrangement process.