The statistical nonequilibrium thermodynamic theory of diffusion-influenced reactions is employed to study reversible excimer formation in solution. Three types of rate coefficients for bimolecular excimer formation are discussed : (i) molecular rate coefficients defined by one-way reactive fluxes, (ii) phenomenological rate constants based on form of the macroscopic rate equations, and (iii) modified, time-dependent phenomenological rate coefficients. Relations are derived linking : (i) the molecular rate coefficients for reversible and irreversible excimer formation rates, and (ii) the steady-state molecular rate constant and the Laplace transform of the time resolved irreversible rate coefficient. The relationship between the present approach and the microscopic-stochastic theory of excimer formation is discussed.