The stepwise ionization (H(3)R(+) <==> H(2)R <==> HR(-) <==> R(2-)) of four fluorescein fluoro derivatives was studied by visible spectroscopy. The pK(a) values were determined in water, in 50 mass % aqueous ethanol, in oil-in-water microemulsions (benzene + CTAB + pentanol-1 in water with 1.0 M KCl; CTAB = cetyltrimethylammonium bromide), and in reversed ones (water + AOT in n-octane; AOT = bis-2-ethylhexylsulphosuccinate or Aerosol OT). The medium effects, DeltapK(a), i.e., changes in pK(a) of these dyes on going from water to some other solvent systems, were rationalized by considering the tautomerism, the values of microscopic ionization constants, and the charge types of the acid-base couples. An expressed shift of the tautomeric equilibria of H(2)R toward colorless lactone was registered on going from water to both aqueous ethanol and organized solutions. While the monoanions HR(-) of 3',4',5',6'-tetrafluoro- and 2,7,3',4',5',6'-hexafluorofluorescein exist in all the systems studied as a tautomer with ionized carboxylic and nonionized hydroxy groups, in the case of 2,4,5,7-tetrafluorofluorescein, the prevalence of another tautomer was observed (COOH and O(-) groups). For 2,7-difluorofluorescein (Oregon Green 488), the partial shift of the tautomeric equilibrium of HR(-) was registered from (COO(-) and OH) in water to (COOH and O(-)) in other solvent systems. The data for the dyes located in an AOT-based pseudophase indicate that the interior of the latter exerts essential differentiation of the acid strength of the dyes, probably caused by the peculiarity of dye species location in water pools. While the state of tautomeric equilibria resembles that in nonaqueous media, the absorption maxima of R(2-) species are close to those in water. Such nonuniform influence displayed by AOT-based water droplets should be taken into account when examining them by using different molecular probes.