Both trans and cis isomers of 4-amino-3-halogenobut-2-enoic acid have been prepared as potential irreversible inhibitors of the enzyme GABA- transaminase. trans-Addition of HX to 4-chlorobut-2-ynoic acid and subsequent amination gave the trans isomers (2; X = Cl, Br, I), while the key step in the synthesis of the cis isomers (3; X = Cl, Br) was the isomerization to cis-4-bromo-3-halogenobut-2-enoic acids during allylic bromination. The stereochemical assignments are supported by ¹H and ¹³C n.m.r. spectral data. A convenient preparation of cis-4- aminobut-2-enoic acid by reduction of the bromo derivative is described, as well as the synthesis of 4-phthalimidobut-2-ynoic acid which is suitable for preparing radiolabelled GABA of high specific activity.