We have used microcalorimetry to examine the stoichiometric adsorption complexes formed by ammonia and pyridine at the Bronsted-acid sites in three samples of H-ZSM-5 which vary widely in aluminum content and synthesis methods. The results show that the heats of adsorption for both ammonia and pyridine are constant up to a coverage of one molecule/Al, independent of Si/Al ratio, with values of 145 kJ/mol for ammonia and 200 +/- 5 kJ/mol for pyridine. These results suggest that the Bronsted-acid sites in unsteamed H-ZSM-5 are independent of sample and equal in concentration to the framework aluminum content.