Luminescence life time measurement can be used under certain conditions to determine the thermodynamic constants of complex formation between the luminescent (central ion) and ligand. The basic equations correlating the life time and the equilibria constants were derived for two cases: the time for establishing the thermodynamic equilibrium is much shorter than the life time of the excited state of the central ion; and the time for establishing the equilibrium and the life time are of the same order of magnitude.