The determination of reactive adsorption mechanisms on metal–organic frameworks remains largely unexplored and knowledge in that field would provide an important stepping stone in enhancing the performance of these materials for gas separation. In this study, the mechanisms of ammonia adsorption on HKUST-1 and its composite with graphite oxide (GO) were analyzed using microcalorimetry, and the results were compared to those derived from other characterization tools. The key to this study lies in conducting measurements at very low surface coverage in order to define the most energetic adsorption phenomena. It was found that ammonia reacted with the Cu sites and then with the ligands causing MOF to collapse. On the other hand, GO enhanced the heats of adsorption owing to its additional reactive sites.