Seven wetlands from two dune systems on the Swan Coastal Plain, Western Australia were studied for sediment properties associated with phosphorus retention and potential remobilization. Sediments from the Spearwood Dunes had higher contents of silt, clay, and organic material. Sediment total phosphorus was positively correlated with water content, organic matter and silt content but inversely related to the amount of coarse materials. Part of the organic matter was refractory humic substances, and the molecular weight of humic acids ranged from less than 10,000 to over 600,000. Significant portions of sediment total P (5–73%) were bound to humic substances. The fraction of “loosely-bound phosphorus” (NH₄Cl-P) accounted for less than 10% of total-P. NaOH-extractable P was high; some humic substances were extracted with NaOH. Correlations between phosphorus forms and other components suggest an affinity between ‘fine particles’ of similar hydrologic density during sediment transport and reworking, and this involves most humic-P in these wetlands.