This thesis describes investigations centred on the mechanistic pathway of the Lewis acid-promoted conversion of epoxides into aldehydes. Such studies were directed to gain new information about this important, fundamental rearrangement which contributes to our knowledge and understanding of how and why chemical reactions occur. The research undertaken involved the use of deuterium labelling studies working with enantio-enriched epoxides. Chapter One reviews previous literature and the results of previous mechanistic studies into the Lewis acid catalysed rearrangements of epoxides. Chapter Two describes the development of new methodology for the measurement of the relative amounts of hydrogen and deuterium in each position of the aldehyde produced by hydride or deuteride migration in the rearrangement of styrene oxide. The method consisted of reduction of the aldehyde to an alcohol and subsequent reaction of the alcohol with a chiral acid chloride in the presence of a chiral shift reagent [Yb(hfc)3 or Eu(hfc)3] to measure the diastereomeric/enantiomeric excess in the aldehyde product mixture. The diastereomeric/enantiomeric excess thus determined, can be correlated back to determine the full stereochemical course of the initial epoxide rearrangement. De-convolution of overlapping peaks by mathematical modelling was sometimes necessary. Chapter Three describes the enantiose1ective synthesis of several undeuterated and deuterated styrene oxides that were then used as substrates to study their BF3.OEt2 and LiClO4 rearrangement to give aldehydes. In Chapter Four the BF3.OEt2 catalysed rearrangement of enantioenriched deuterated analogues of styrene oxide are investigated. Chapter Five describes the difficulties encountered when attempting to rearrange both racemic and optically active styrene oxides with LiClO4 in refluxing benzene. Dr D.Q. McDonald's reactions are reproduced to substantiate the claim that his previously reported results were compromised by traces of acid present in the samples of epoxide starting material. The LiClO4/ether catalysed rearrangement of enantio-enriched deuterated analogues of styrene oxide are investigated in Chapter Six with evidence of some racemisation occurring during the rearrangement process. An overall discussion is presented in Chapter Seven with results of the rearrangements of styrene oxide. The results are interpreted using both the Blackett and Fujimoto models and comparisons are made with previous mechanistic studies. Explanations and final conclusions are drawn with future studies outlined. The experimental details that substantiate the claims made in the previous chapters are included in Chapter Eight.