Iron oxide (Fe2O3) was identified and characterized by surface area, X-ray diffractometry, and FTIR analyses. Surface charge densities, point of zero charge (PZC), and surface ionization constants were determined from the potentiometric titration data in various aqueous and aqueous organic mixed solvents in the temperature range 293-313 K. The surface charge densities were observed to decrease with the increase in temperature and concentration of metal ions in both the aqueous and aqueous organic mixed solvents. The absolute values of the surface charge density were found to change in the order aqueous > aqueous/methanol > aqueous/ethanol. Further, the PZC of the iron oxide was observed to shift to the higher pH values in the order ethanol > methanol > aqueous solution, which indicated a decrease in the acidity of the surface -OH groups. The pK(a1) and pK(a2) values of iron oxide were also determined and then used for determination of the surface potential (psi(0)) of the solid in aqueous and aqueous organic mixed solvents. The surface potential-surface charge curves generally supplemented the results derived from psi(0)-pH curves. (C) 2004 Elsevier Inc. All rights reserved.