The positions of nu8a and nu*(NH) bands in the spectrum of protonated 2,6-dimethylpyridine vary with the strength of Brønsted acidity: the higher the nu*(NH) wavenumber and the lower the nu8a wavenumber, the stronger the acidity. The results obtained with 2,6-dimethylpyridine adsorption correlate with those obtained by CO adsorption experiments on a series of faujasite zeolites (LiHNaY, KHNaY, HY, HY(SA), HNaX). These relations were extended to gamma-Al2O3 having weak Brønsted acidity, not detected by pyridine and hardly detected by CO. The number (0.1 per nm2) and the strength (corresponding to delta nu (OH) by CO = 155 cm(-1)) of the most acidic OH groups of Al2O3, as well as the position of the corresponding nu (OH) band (<3700 cm(-1)) are deduced from 2,6-dimethylpyridine adsorption experiments.