Reaction mechanisms of amine compounds with ozone were investigated with 18O isotope labeling of the final products, nitrate and aldehydes (or ketones), for varying concentrations of 18O-labeled water. For oximes and gabapentin, nitrate and aldehydes showed an increased 18O isotopic ratio/composition for higher proportions of 18O-labeled water, while no effect was observed for amino acids, peptidic amino groups, and aliphatic (or aryl-type) primary amines. Gabapentin might react through an oxime intermediate involving oxygen exchange with water during the ozone reaction. In contrast, amino acids, peptidic amino groups, and aliphatic (or aryl-type) primary amines might have nitroalkanes as common reaction intermediates, which do not involve oxygen exchange with water during ozone transformation reactions. The 18O isotopic composition of aldehydes was significantly influenced by oxygen exchange through hydration, even after the ozone reaction was complete. Thus, 18O isotopic composition of aldehydes is unsuitable for mechanistic studies. In contrast, formation of ketones and carboxylic acids was not affected by hydration, indicating that their 18O isotopic composition could be used to assess reaction intermediates. However, since aldehydes are typical precursors to carboxylic acids during ozonation, the source of oxygen in carboxylic acids remains inconclusive due to the hydration of aldehydes. Overall, the use of 18O-labeled ozone or water for elucidating ozone-induced reaction mechanisms cannot be generally applied and has to be critically evaluated for each reaction system.