Ion-selective electrodes are often used in solutions which contain interfering ions. Calibration under these conditions is usually achieved by the fixed interference method, in which the concentration of the test ion is varied in a fixed background of interfering ion. The present paper describes a method for analyzing fixed interference calibration data which allows the Nicolski coefficients to be extracted using commercially available computer software. The method produces an accurate numeric description of the calibration curve. It also enables an accurate correction for the situation where the experimental level of interfering ion is not identical to that of the standard solutions.