The 1H NMR analysis of leptospermone in acidified D2O displays four different signals in the 0–3 ppm region; two doublets at 0.9 (6H) and 2.8 ppm (2H), a singlet at 1.4 ppm (12H) and a multiplet at 2.0 ppm (1H). These data were compared to the predicted chemical shifts for the keto and the two enol forms (enol on the lateral chain or enol on the cycle). The methyl groups located on the AG 825 are only equivalent for the keto and enol on the lateral chain structures giving a singlet centered respectively at 1.3 and 1.4 ppm. Therefore, the enol on the cycle is unfavored in acidified heavy water. The discrimination between keto and enol on the lateral chain structures is made difficult because of the overlapping of signals with those of water. However, the two protons linked to the carbon atom in α position to the lateral CO appear at 2.8 ppm, against predicted values of 2.4 and 1.9 ppm for the keto and enol, respectively. Therefore the keto structure may be the dominant species.