The acid-base equilibria of fleroxacin were studied by means of potentiometry and spectrophotometry. It was established that fleroxacin undergoes a complex acid-base equilibrium due to its zwitterionic nature and two proton-binding sites of similar acidity. The stoichiometric equilibrium constants were determined at 25 degrees C and constant ionic strength 0.1 M (NaCl). The acidity constants pK(1) = 5.59 +/- 0.01 and pK(2) = 8.08 +/- 0.04 were found by potentiometry, and pK(1) = 5.61 +/- 0.03 and pK(2) = 8.11 +/- 0.06 by spectrophotometry. The distribution diagram of the corresponding ionic species is given. (C) 1998 Elsevier Science B.V. All rights reserved.