Adsorptions of N-2 and CO on H-ZSM-5 at low temperature are examined in sequence by means of transmission FT-IR spectroscopy, which enables the direct comparison of N-2 and CO as probes of acid sites. Specific adsorptions of N-2 and CO on Bronsted acid sites (BAS) and Lewis acid sites (LAS) are distinguished by their characteristic absorption bands at 2334 (BAS) and 2351 cm(-1) (LAS) for N-2 and at 2177 (BAS) and 2227 cm(-1) (LAS) for CO, respectively. Previously reported assignments of these bands are confirmed by the cm results of the N-2 and CO adsorption mutually. Relative molar extinction coefficients, epsilon, of the adsorbed N-2 and CO species are determined as follows : epsilon(CO on LAS) approximate to 2.6, epsilon(CO on BAS) approximate to 8.1, epsilon(N-2 On BAS) approximate to 0.50, assuming that epsilon(N-2 on LAS) is unity. Other characteristics of N-2 and CO as probes of acid sites are discussed.