The acidity variations upon Si/Al substitution at NNN sites were calculated for mordenite by the ab initio molecular orbital method. The conclusions obtained from our previous work on faujasite hold with mordenite, e.g., the effects of Si/Al substitution at NNN sites on the acid strength are not uniform. Mordenite was synthesized with various Si/Al ratios and the acidic properties were examined by means of ammonia adsorption calorimetry. The analysis of the experimental results based on the binomial distribution of Al indicates that not only ONNN sites but some of 1NNN, 2NNN, etc. should also be strong acid sites, which is consistent with the calculation.