In the presence of the tabun mimic diethylcyanophosphonate (DECP), a fluorescent bifunctional coumarin–enamine chemodosimeter is first phosphorylated and subsequently attacked by the released cyanide ions. The reaction is disclosed in both the UV-vis and fluorescence spectra and confirmed by NMR experiments. DFT calculations support that the bis-adduct as the most thermodynamically stable species. The X-ray structure of the chemodosimeter is also reported.