From steady state kinetics of a general excited state reversible reaction scheme, a linear relation between the total luminescence and the concentration of one reactant is derived, showing that the equilibrium constant of certain excited state reactions can be determined from the total fluorescence spectra without the separation of the components and irrespective of the ground state equilibrium. The relation is checked by comparison with published results, and it is applied to the determination of excited state acidity constants of acridone, xanthone and thioxanthone in ethanol-water (1:1).