A luminescence-optical indicating substance for continuous and reversible measurement of the pH value of a sample within a measuring range M of interest, said sample being at least in indirect contact with said indicating substance provided as a base B*, having a luminescence decay time .tau. depending on the pH value of said sample, comprising
- a) a pK.sub.a value, in an electric ground state as S.sub.0, which lies in said measuring range M of interest;
- b) a protonation rate k.sub.p of said base B* and a deprotonation rate k.sub.dp of conjugate acid BH.sup.+* in an excited state S.sub.1 lower than a decay rate k.sub.A of said excited state;
- and
- c) luminescence decay times .tau..sub.S and .tau..sub.B of said conjugate acid BH.sup.+* and said base B* which differ from each other.