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Measurements of absolute aggregation rate constants were performed for two samples of well-characterized spheroidal hematite particles with rather different surface morphology. At high ionic strength, the system is in the fast aggregation regime with pH independent rate constants around (1–4) × 10⁻¹⁸m³/s. At low ionic strengths, the rate constant is a function of pH and goes through a flat maximum around the point of zero charge (PZC) where fast aggregation conditions are reached. With increasing pH the rate constants increase gradually below the PZC while they decrease very rapidly above the PZC. Above the PZC the rate constants are rather well predicted by the classical DLVO theory. Below the PZC, however, it is necessary to assume a distribution in the surface potentials with a coefficient of variation around 30% to account for the observed behavior. In spite of the pronounced differences in the surface …