Synthetically prepared maghemite and fluorapatite, characterized with BET, SEM, XRD, FT-IR, and FT-Raman, are used to investigate the protolytic properties and surface characteristics in a mixed system of maghemite and fluorapatite by means of potentiometric titrations and surface complex modeling. Titrations were performed in the pH range of 7.3-10.5 at 25 +/- 0.2 degrees C in ionic media of 0.10 mol dm(-3) NaNO3 with 0.0100 mol dm(-3) HNO3 and 0.0100 mol dm(-3) NaOH used as titrants. The constant capacitance model (CCM) was applied to interpret the titration data. Two models with different surface equilibria were tested. In the first model, the mixed system was treated as a one-component system with a total surface area of 40.04 +/- 5.2 m(2) g(-1) without any consideration to the subsystems. The surface equilibria, &3bond; XOH + H+ reversible arrow &3bond; XOH2+, Ig beta(11)(s)(int) = 6.74 +/- 0.07; &3bond; XOH reversible arrow &3bond; XO- + H+, Ig beta(-11)(s)(int) = -7.75 +/- 0.07, were found to represent an accurate model for the system, and the specific capacitance was optimized to 2.0 F m(-2). The number of active surface sites N-s was found to be 1.2 sites nm(-2). This model has, however, no relation to the subsystems of maghemite and fluorapatite. The second model is related to the subsystems and displays the surface equilibria, &3bond; S2OH reversible arrow &3bond; S2O- + H+, Ig beta(-101)(s)(int) = -9.12 +/- 0.01; &3bond; FeOH + H+ reversible arrow &3bond; FeOH2+, Ig beta(11)(s)(int) = 6.80 +/- 0.01; &3bond; FeOH reversible arrow &3bond; FeO- + H+, Ig beta(-11)(s)(int) = -7.77 +/- 0.01, where =-S2OH is related to fluorapatite and &3bond; FeOH is representing maghemite. Fluorapatite corresponds to the dominating active surface in the system. The specific capacitance was optimized to 18 F m(-2). The N-s values were found to be 2.27 sites nm(-2) for fluorapatite and 0.80 sites nm(-2) for maghemite. The N-s values together with evidence from the FT-Raman and SEM investigations reveal that interactions between maghemite and fluorapatite surfaces occur during the titration. The acid-base properties and surface characteristics of the subsystems maghemite-H+ and fluorapatite-OH- using the CCM have been published earlier.