In highly weathered soils, such as tropical soils, organic phosphorus (P) may decline together with total P and could be represented by a relatively lower amount of inositol phosphates compared with temperate soils. The aim of this work was to understand the abiotic processes leading to inositol phosphate retention in highly weathered soils. Sorption of myo-inositol hexakisphosphate and inorganic phosphate (Pi) on two clay fractions (A and B) extracted from Brazilian oxisols was studied, examining both the extent of P retention and surface property modifications induced by anion sorption. The two clays were characterized by a high amount of crystalline iron oxides and kaolinite. In clay A, gibbsite was also present, plus a higher amount of organic matter. The two clays presented a similar specific surface area, but clay A had a lower microporosity. The amount of Pi sorbed on the two clays was similar, reaching a plateau at about 1.35 2mol m j2 , whereas the amount of myo-inositol hexakisphosphate was higher on clay A, 0.67 2mol m j2 , than clay B, 0.49 2mol m j2 . The retention of both P forms on the two clays was linked to the large presence of crystalline Fe and Al oxides. Whereas Pi adsorbed on the external surfaces and diffused into micropores, myo-inositol hexakisphosphate likely adsorbed only on the external surfaces. The precipitation of Al and Fe myo-inositol hexakisphosphate was also limited because of the small amounts of soluble Al and Fe forms. All these phenomena may contribute to justify the relatively low accumulation of these organic P compounds found in some highly weathered soils. (Soil Science 2008;173:694-706)