The determination of chloride ion in soil extracts for cation exchange capacity in the presence of ammonium ions of nearly equal concentration (∼5 x 10‐2M) and excess sodium sulphate (0.5 M) by an Orion solid state selective ion electrode is reported. Ammonium ions were found to interfere by reducing the determined value of the chloride ion by up to 20%. This interference effect was compensated by the determination of a mean activity coefficient for ammonium chloride in the sodium sulphate medium. This method was accurate to ±7%. No interference effect was found for the chloride ion determination of a mixed sodium chloride‐sodium sulphate solution.