A potentiometric sensor for the perchlorate anion was developed by mixing chemically modified silicagel with pyridinium perchlorate, with an epoxy polymer and graphite. The electrode showed Nernstian response between 1.0 x 10(-2) and 1.0 x 10(-3) M perchlorate concentrations. The electrode showed high selectivity to this ion at solutions pH between 5.5 and 8.0. The presence of IO4-, NO3-, Br-, IO3-, Cl- and SO4(2-) ions in the solutions, had only small interference in the electrode response in the range mentioned.11741732239244