The interfacial behavior of two hydrous oxides of iron, namely an amorphous variety (amor. (\mathrm{FeOOH}) ) and a synthetic akaganeite ( (\beta) - FcOOH), in different electrolyte media has been studied by potentiometric titration (both rapid and batch procedures) and particle electrophoresis methods. The background electrolyte medium was varied from simple (\mathrm{KNO}{3}) and (\mathrm{NaCl}) solutions to a complex electrolyte solution resembling seawater. In (\mathrm{KNO}{3}) solution the (\mathrm{pH}{\mathrm{pzc}}) and the (\mathrm{pH}{\mathrm{icp}}) are almost the same for both samples. In (\mathrm{NaCl}) solution, the (\mathrm{pH}{\text {ier }}) shifts by about a (\mathrm{pH}) unit toward the acid side for amor. (\mathrm{FeOOH}) due to specific sorption of anions; for (\beta) - (\mathrm{FeOOH}) the (\mathrm{pH}{\text {iep }}) shifts slightly toward the alkaline side due to the small specific sorption of cations. Consequently, the (\mathrm{pH}{\mathrm{rx}}) of amor. (\mathrm{FeOOH}) in (0.5 \mathrm{M} \mathrm{NaCl}), although it shifts slightly to the acid side in the presence of (\mathrm{MgCl}{2}), has no corresponding shift to the alkaline side observable in the presence of (\mathrm{Na}{2} \mathrm{SO}{4}) solution. Calculations for surface species in different electrolyte media suggest that while surface ionization is stronger than complexation for amor. (\mathrm{FeOOH}), surface complexation is the stronger phenomenon contributing to the total surface charge on (\beta-\mathrm{FeOOH}). All these observations are in conformity with the (\Delta \mathrm{p} K_{\mathrm{a}}^{\mathrm{int}}) value of amor. FeOOH being smaller than that of (\beta)-FeOOH. (i) 9994 Academic Press. Inc.