The acid-base characteristics of the manganite (gamma-MnOOH) surface have been studied at pH above 6, where dissolution is negligible. Synthetic microcrystalline particles of manganite were used in the experiments. From potentiometric titrations, electrophoretic mobility measurements, and X-ray photoelectron spectroscopy (XPS), a one pK(a) model was constructed that describes the observed behavior. The data show no ionic strength effect at pH < 8.2, which is the pH at the isoelectric point (pH(iep)), but ionic strength effects were visible above this pH. To explain these observations, Na+ ions were suggested to form a surface complex. The following equilibria were established: =MnOH2+1/2 reversible arrow =MnOH-1/2 + H+, log beta(0) (intr.) = -8.20; =MnOH2+1/2 + Na+ reversible arrow =MnOHNa+1/2 + H+, log beta(0) (intr.) = -9.64. The excess of Na+ at the surface was supported by XPS measurements of manganite suspensions containing 10 mM NaCl. The dielectric constant of synthetic manganite powder was also determined in this study.