The neutral lipophilic chromoionophore ETH 5294 is characterized in solvent polymeric membranes with a transient potentiometric method on polarized segmented sandwich membranes. The pK(a) value obtained, with standard deviation, is 11.1 +/- 0.1, which corresponds reasonably well with the previously estimated pK(a) approximate to 12.0 in the same polymer matrix. In contrast, the complex formation constant with potassium is extremely small, with a logarithmic value of 1.51 +/-0.02, indicating a weak binding strength in the sensing membrane. The method proposed makes the characterization of a variety of neutral lipophilic H+-selective chromoionophores in situ experimentally accessible. The information obtained is not only important for the design of carrier-based optodes and pH electrodes, but also gives direct quantitative evidence that such a chromoionophore can be used as a reference ionophore in a potentiometric approach to determine ion-ionophore complex formation constants.