Crystallographic analysis of the bromo-beta-lactones obtained by addition of bromine to aqueous solutions of disodium 2,3-dimethylmaleate and 2,3-dimethylfumarate reveals stereochernistries (4 from 1, and 3 from 2) opposite to those originally assigned (3 from 1, and 4 from 2). Specifically, the maleate leads to a bromo-beta-lactone with the methyl groups in a trans relationship, whereas the fumarate leads to the corresponding cis isomer. To account for this observation, we suggest that the first-formed intermediate in each case is an alpha-lactone. B3LYP/6-31 + G(d) calculations in PCM water indicate that the cyclic chloronium and bromonium adducts of acrylate anion are not intermediates but transition structures for the degenerate rearrangement of halomethyl-alpha-lactones. Bader analysis of MP2/6-31 + G(d,p) electron density distributions indicates that oxiranone possesses considerable ionic character in the endocyclic C-alpha-O-n bond. In PCM water there is neither a ring critical point nor a bond critical point for C-alpha-O-n, although geometrically the molecule still possesses an acute-angled three-membered ring with a C-alpha-C-n angle of only 69degrees. Combined quantum/classical calculations for B3LYP/6-31 + G(d) oxiranone surrounded by about 600 explicit TIP3P water molecules indicate that the cyclic structure is an energy minimum in aqueous solution, and Bader analysis gives a result similar to that from the continuum model. Copyright (C) 2002 John Wiley Sons, Ltd.