An effect which occurs when ion-selective electrodes are used in concentrated, space-filled suspensions while the external @R05229@ remains in the supernatant (@S06198@-free) 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 @S06198@ differs from the value in the supernatant by the interfacial PD, and corresponds to a higher value approximating the activity inside the @I03171@ @G02600@. The effect nearly disappears when the outer @R05229@ is placed in the same region of the @S06198@ as the sensor electrode. There are some changes in the junction potential differences of the @R05229@, between @S06198@ and supernatant.