The isomerization of dimethyl 2,3-dibromofumarate in chloroform solutions was investigated by the combination of nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations. The bromination of dimethyl acetylenedicarboxylate leading to dimethyl 2,3-dibromofumarate produces the trans isomer initially, which however converts into the more stable cis isomer. The conversion from trans to cis is spontaneous and greatly accelerated by light.