The effects of the Si --> Al substitution on the structure of the siliceous framework and the structure and vibrational properties of Bronsted acid sites in mordenite have been studied using density-functional theory, both in the local-density approximation and including generalized gradient corrections to the exchange-correlation functional. The substitution induces a substantial local deformation of the polytetrahedral geometry of pure-silica mordenite. Also the counterions have a strong influence on the geometry of the framework. Protonation of bridging (Si)-O-(Al) oxygen atoms is accompanied by a further local distortion of the structure. Changes in the bond lengths remain well localized to the nearest neighborhood of the perturbation center; O-H stretching frequencies of acidic protons were calculated in both harmonic and anharmonic approximations, indicating a rather complex relationship among stability, frequencies, and local environment of the Bronsted acid sites.