The adsorption of hydrogen ions on precipitated hydrous FePO(4)(am) (HFP) was studied. The experiments were carried out in 0.3 mol.dm(-3) KNO(3) solutions and NaClO(4) solutions at ionic strengths of (0.002, 0.010, 0.030, 0.1, 0.3, and 1.0) mol.dm(-3). The acidity of surface ferrihydrols, >FeOH, and phosphohydrols, >POH, groups was evaluated from potentiometric acid-base titration curves. The pH values ranged from 2.5 to 5.5. The adsorption equilibria were found to be slow but showed good reversibility. Effects by medium ions were studied using two amorphous hydrated solids obtained with different procedures: (a) precipitation and washed by dialysis (HFP(dialyzed)); (b) precipitation in situ in the presence of the ionic medium in question HFP (in situ). On the exchange of protons at the interface HFP solution, a. charged surface results with positively, >FeOH(2)(+), and negatively, >PO(-), charged sites. The charge is a function of the solution acidity and is regulated by the acid-base equilibria: > FeOH(2)(+) reversible arrow > FeOH + H(+) log(Fe) K(a1) = -4.66 +/- 0.05 > POH reversible arrow > PO(-) + H(+) log(P) K(a2) = -1.4 +/- 0.1 The zero charge point (ZCP = 3.04 +/- 0.05) and the number of sites were determined.