The pK a constant of varenicline using a partial agonist of the nicotinic α4β2 acetylcholine receptor was determined potentiometrically. The pK a constant of varenicline was calculated as 9.22 (± 0.13) at I = 0.15 (NaCl), 25 ± 1 °C using the Irving–Rossotti method. The experimental results were compared with theoretical ones obtained via the SPARC pK a calculation program. The distribution of species of the drug molecule at various pH values was demonstrated to provide experimental data of its pK a, to be used in commenting on its biological uptake, the binding of these molecules to environmental matrices, forming chelates with metallic cations, and its analysis.