A series of dealuminated hydrogen mordenites (total Si/Al = 7.7, 11.6, 16.8, 29.0, 39.0) were investigated by diffuse reflectance IR spectroscopy with and without probe molecules (CO, H2). In the stretching vibration region of the pure samples bands were observed at 3613–3611, 3734, 3675 and 3510 cm–1 at room temperature and assigned to bridging OH groups [νAl(OH)Si], silanol groups (νSiOH), OH of Al-containing species partially coordinated to the framework and hydrogen-bonded SiOH species in highly dealuminated specimens, respectively. When the IR spectra were obtained at 77 K, both the bands of νAl(OH)Si and νSiOH shifted to higher wavenumbers, viz. 3619 and 3740 cm–1, respectively. In the region of combination modes, bands of the pure samples were detected at 4658–4650, 4565–4552 and ca. 4500 cm–1 which could be ascribed to νAl(OH)Si+δAl(OH)Si, νSiOH+δSiOH and νSiOH+νSiO, respectively. Upon interaction with H2 or CO at 77 K the following shifts were measured: ΔνAl(OH)Si/H2= 53–63 cm–1; ΔνSiOH/H2= 5 cm–1; Δ[νAl(OH)Si+δAl(OH)Si]/H2= 24–31 cm–1; Δ(νSiOH+δSiOH)/H2= 0; ΔνAl(OH)Si/CO = 359–389 cm–1 and ΔνSiOH/CO = 102 cm–1. The intensities of the bands due to νAl(OH)Si and νAl(OH)Si+δAl(OH)Si decreased with increasing dealumination as a result of removal of acidic bridging OH groups. By contrast, the shifts Δ[νAl(OH)Si+δAl(OH)Si]/H2 and ΔνAl(OH)Si/H2 continuously increased with the degree of dealumination which is proposed to indicate a corresponding increase in the strength of the Brønsted centres, Al(OH)Si, in line with the earlier TPD results obtained with the same samples. Bands, which appeard upon H2 interaction at 77 K around 4070 and 4020 cm–1, are discussed in view of findings reported in the literature for similar systems and tentatively assigned to ‘true’ Lewis-acid sites (removable by acid leaching) and to only threefold-coordinated Al and/or Si of the framework, respectively. Finally, the bands of H2 perturbed by silanol groups (4103 cm–1), of physisorbed CO (4250 cm–1), the effect of dealumination on the formation of defects and their interaction with the probe molecules as well as the problem of the influence of CO pressure on the band shift ΔνAl(OH)Si/CO are discussed in detail.