We seek to understand the pH-dependent physicochemical changes of lonidamine (LND), an antineoplastic drug, particularly to locate the sites of ionization. LND samples at pH 2, 7, and 13 were analyzed using 1H and 13C NMR. The results indicate that there is a noticeable change in the chemical shifts for a few atoms in LND from neutral to alkaline pH. These changes demonstrate that LND is ionized at its imidazole α-nitrogen. In addition, the expected ionization of the carboxyl group of LND at acid pH is not directly observed, and this may be due to a rapid-exchange phenomenon.