We have developed a methodology that enables for the rapid measurement of ionization constants (pK a) of series of compounds by UV spectrophotometry. This protocol, which is straightforward to set up, takes advantage of the sensitivity of UV spectroscopy and the throughput enabled by the 96-well microplate (as opposed to the use of 1 cm quartz cuvette). The compounds, in stock solutions in DMSO, are dissolved in several aqueous buffer solutions directly in the microtiter plate, allowing the simultaneous determination of the UV spectra as a function of pH. Further treatment of the data provides the pK a values in a medium-throughput manner. The pK a values of 11 new antitrypanosomal dibasic compounds were determined using this methodology.