Surface characteristics of magnetite at 25 degrees C in aqueous suspensions are systematically studied. The measured specific surface area and concentration of proton binding sites correspond well to the calculated values. Zeta potential of magnetite in the absence of multivalent cations exhibits positive values in acidic solution and becomes negative with increasing pH. pH(pzc) is about 6. In the presence of excess cations such as Fe2+ or Fe3+, specific adsorption takes place at the surface of magnetite, which dramatically influences the value of zeta potential. The concentrations of soluble Fe ions at pH about 4.5 increase with solid concentrations of magnetite indicating some surface-related reaction mechanisms.