The basic charging proper-ties of nearly spherical hematite particles were studied by using potentiometric titration and the electroacoustic technique. Both the pH and the ionic strength dependence of the surface charge and the zeta -potential were studied in detail. For calculating the zeta -potential from mobility data a few different theories,were used and obtained differences are discussed. At pH values higher than 7 and at high electrolyte concentrations (50 mM and 100 mM NaNO3), it was difficult to fit the mobility data by using the full mobility spectra including both magnitude and phase angle at several frequencies. In this regime the best fits were obtained by using a theory for aggregated complexes (porous particles). From potentiometric titrations in 0.01, 0.1, and 1.0 M NaNO3, parameters fora 1-pK Basic-Stem Model were determined. The model was used to examine the possibility of correlating the experimentally determined zeta -potentials to the model-calculated potentials at the Stem plane. Qualitatively, the model predicted the correct ionic strength dependence of the zeta -potentials, and there was also a rather good quantitative agreement at high ionic strengths (50 and 100 mM NaNO3). However, at lower ionic strengths the model predicted values up to 40% higher than those found from the electroacoustic study. Surface conduction behind the slip plane was discussed as a possible cause for this discrepancy.