A potentiometric method for the determination of the apparent mean activity of NaCI in saline soils in situ was proposed. A spear-type sodium-selective glass electrode and a spear-type chloride-selective pressed electrode were used. It was found that under ordinary circumstances the equilibrium potential between the electrodes and the soil generally reached within 10 min. after the insertion of the electrode into the soil, the actual time being dependent on the water content and NaCI content of the soil. For most saline soils with a water content near field capacity, the equilibrium time was 2-5 minutes. Prolonged usage may affect the function of the electrodes due to the adhesion of soil colloidal particles on the surface of the chloride-selective electrode and the dehyciration of the sodium-selective electrode. These affects may be avoided by frequent rubbing with fine emery paper and rinsing with water respectively.