The fluorimetric determination of the ground-state dissociation constant K-d of intermolecular bicompartmental systems in the absence and presence of an excited-state reaction is discussed. The presence of an excited-state reaction can result in very complicated fluorimetric titration curves, precluding the evaluation of K-d. The interference of the excited-state reaction can be circumvented if the fluorimetric titration is performed at the isoemissive point. Under this condition, the correct value of K-d Will be obtained from the unique inflection point of the titration curve. It is further shown that previous recommendations to excite either at the isosbestic point or at a wavelength where only one species absorbs are inappropriate.