Structural and acidic properties of the (001) surface of mordenite have been studied using density functional theory including generalized gradient corrections to the exchange-correlation functional. Our calculations, in agreement with experimental results, show that the surface structure of mordenite differs only moderately from the bulk structure, no reconstruction of the surface has been found. Part of the terminal silanol groups form weak hydrogen bonds with other framework oxygen sites, the lengths of hydrogen bonds vary in the range 1.9-2.6 Angstrom depending on the position of the Al site. The OH-stretching frequencies for various acid sites in the mordenite framework have been calculated. In agreement with experiment, calculated OH-stretching frequencies of terminal silanol groups are shifted by about 100 cm(-1) with respect to the Bronsted acid sites. Those silanol groups whose OH-stretching frequencies are modified by the presence of hydrogen bonds are shown to absorb in the region typical for the Bronsted acid sites.