An effect which occurs when ion-selective electrodes are used in concentrated, space-filled suspensions while the external @[email protected] remains in the supernatant (@[email protected]) solution. The suspensions are specifically solvent-swollen ion exchangers or other materials, like soils and clays, that concentrate ions by adsorption and absorption. Space-filled, gravity-packed suspensions act like a second phase and form apparently an interfacial potential difference (PD) with respect to the supernatant. The measured ion activity in the @[email protected] differs from the value in the supernatant by the interfacial PD, and corresponds to a higher value approximating the activity inside the @[email protected] @[email protected] The effect nearly disappears when the outer @[email protected] is placed in the same region of the @[email protected] as the sensor electrode. There are some changes in the junction potential differences of the @[email protected], between @[email protected] and supernatant.