Anion sensor properties of N-alkyl-substituted 1,4 '-diazaflavonium bromides in methanol-water were evaluated by UV-vis spectrometry. Pronounced changes were observed in the absorption spectra of all compounds for only OH-, CO32-, and CN- among F-, Cl-, Br-, I-, OH-, CO32-, NO3-, PO43-, CN-, SO42-, HSO4-, HCO3-, SCN-, NO2-, and P2O72- ions. Two new absorption bands at 385 and 685 nm accompanying the distinct color change for OH-, CO32-, and CN- ions were observed in case of all compounds. The color changes were from pink to blue for CO32- and OH- ions and from pink to purple for CN- ion. Thanks to the distinct color change, the compounds can be used as selective colorimetric anion sensors. Linear changes of absorbance of N-heptyl-substituted compound at 385 nm as a function of the ion concentration were used to determine CN- ion in water samples. Detection and quantification limits of the proposed method were 0.94 and 2.82 mg/L, respectively.