3-(α- Bromobenzyl )-3-methoxypiperazine-2,5-diones are prepared by the reaction of 3-benzylidenepiperazine-2,5-diones with N- bromosuccinimide in methanol. The crystal structure of the threo-1-acetyl compound shows the side chain in the EN conformation, with the bromine above the heterocyclic ring, the latter ring system being almost planar. This conformer predominates in CDCl3 solution also, but in (CD3)2SO the piperazine-2,5-dione ring adopts a boat conformation. Both the erythro isomer and an N(4)-methyl derivative exist in solution predominantly as folded conformers. The importance of repulsive interactions in predicting the conformation of substituted piperazine-2,5-diones is discussed.