In solutions containing some urea or thiourea derivatives and alkali metal hydroxides at pH 6 to 10, the pH value measured with glass electrodes changes with time. The degree of change varies between 0.1 and 1.0 pH units and depends on the structure of the urea derivative, its initial concentration, and the glass electrode used, but not on the cation of the alkali metal hydroxide added. In contrast, pH measured in the same solution with a quinhydrone electrode or the UV spectra remain time-independent. It is proposed that urea derivatives are adsorbed in the gel at the surface of the glass electrode and there affect the ion-exchange process.