Ion-selective electrodes responsive to maleic and phthalic acids were prepared. The organic solvent solution of the ion-pair of the objective monovalent acid anion with a Crystal Violet or a tris(bathophenanthroline)-iron(II) ion was used as the electrode membrane. The responses of both electrodes were linear down to 10−4–10−4.5 M, with an ideal Nernstian slope of 57–58 mV per activity decade at 20 °C. Together with the potential slope, the pH dependence of the potential showed that the electrode responded to the monovalent anion of maleic or phthalic acid. The interferences of acetate, benzoate, and chloride were extremely low, whereas iodide and perchlorate interfered greatly. The maleic acid electrode works well without the interference of fumaric aicd. The phthalic acid electrode also functions without any interferences by isophthalic or terephthalic acid.