"Base on"/"base off" coordination of the intramolecular bound benzimidazole nucleobase of vitamin B-12 allows the specific colorimetric detection of millimolar concentrations of cyanide in water. In various competition experiments, it was demonstrated that up to 12 different anions as well as a 1000-fold excess of Cl- over CN- do no interfere with the sensor. An 8-fold increased sensitivity in the "naked eye" detection of CN- was observed when water was replaced by MeOH/H2O (5%) as a solvent.