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We have used microcalorimetry to measure the differential heats of adsorption of both a series of alkylamines and a series of substituted pyridines in H-ZSM-5 and H-Mordenite. With few exceptions, the differential heats are approximately constant to coverages close to the expected Brønsted site concentration. Both zeolites show good correlations between the average differential heats of adsorption and the gas-phase proton affinities of the basic adsorbates. Slopes of the correlation lines for the two zeolites are similar; the intercepts differ by about 15 kJ/mol. We use this data set to demonstrate that a self-consistent, quantitative Brønsted acidity scale for solid acids cannot be obtained from heats of adsorption of ammonia or pyridine or any other single reference base. However, the correlation between heats of adsorption and gas-phase proton affinities does provide a useful starting point for a more complete description of the thermochemistry of proton transfer reactions in zeolites. Deviations from the correlation curves for specific zeolite/adsorbate pairs can be used to infer how the strengths of Coulombic, hydrogen-bonding, or van der Waals interactions change with structure of either the zeolite acid or the adsorbate base.