A novel frequency response (FR) method is proposed: (i) Partial pressure(s) of reactant(s) in a steady-flow reactor is perturbed sinusoidally by varying gas space of the reactor with a definite angular frequency ω; (ii) amplitude and phase shift of every partial-pressure variation of both reactant(s) and product(s) is measured over a wide range of ω; (iii) on the other hand, characteristic functions of ω to explain "reaction-rate spectra" obtained from the data in (ii) are deduced analytically on the basis of a reaction mechanism; and (iv) complex rate coefficients, (k + iωl), involved in the functions are determined by numerical simulation to the spectra. The FR method is applied to CO oxidation over Ru/Al 2 O 3 at ca. 10 2 Pa and 623 K in order to confirm its efficiency: (i) As many as thirteen rate coefficients at five elementary steps were determined; (ii) chemical potentials of surface-intermediates, CO(a), O 2 (a), and O(a), were derived from l's; and (iii) free-energy dissipations via the three intermediates were deduced from chemical potential changes at five elementary steps. It is concluded that the reaction sequence of carbon monoxide, CO(g) f CO(a) f CO 2 (g), could occur spontaneously but the other sequence of oxygen, 1 / 2 O 2 (g) f 1 / 2 O 2 (a) f O(a) f CO 2 (g), would be against the reaction, although they are coupled.