It may be advantageous if transient measurements are carried out in the presence of interfering ions in the so-called "two-ion range". The term "two-ion range" indicates that the equilibrium potential is determined by both the primary and interfering ion activities according to the Nicolsky equation. In the presence of interfering ions, the transient functions are expected to be influenced by: the "competition" for the active sites of the electrode surface; the transformation of the electrode surface in time; and diffusion and the chemical reaction within the electrode membrane. Useful practical analytical information can be obtained from response time measurements performed in the presence of interfering ions, since the ratio of the interfering ion and the primary ion can also vary during analytical determinations. The transient functions recorded in the two-ionic range are especially important in flow-through analytical techniques employing ion-selective electrodes (ISE).