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The phosphorus (P) fraction distribution and formation mechanism in the supatant after P adsorption onto iron oxides and iron oxide-humic acid (HA) complexes were analyzed using the ultrafiltration method in this study.With an initial P concentration of 20 mg/L (I =0.01 mol/L and pH =7),it was shown that the colloid (1 kDa-0.45 μtm) component of P accounted for 10.6％,11.6％,6.5％,and 4.0％ of remaining total P concentration in the supematant after P adsorption onto ferrihydrite (FH),goethite (GE),ferrihydrite-humic acid complex (FH-HA),goethite-humic acid complex (GE-HA),respectively.The ＜ 1 kDa component of P was still the predominant fraction in the supematant,and underestimated colloidal P accounted for 2.2％,55.1％,45.5％,and 38.7％ of P adsorption onto the solid surface ofFH,FH-HA,GE and GE-HA,respectively.Thus,the colloid P could not be neglected.Notably,it could be interpreted that Fe3+ hydrolysis from the adsorbents followed by the formation of colloidal hydrous ferric oxide aggregates was the main mechanism for the formation of the colloid P in the supatant.And colloidal adsorbent particles co-existing in the supematant were another important reason for it.Additionally,dissolve organic matter dissolved from iron oxide-HA complexes could occupy large adsorption sites of colloidal iron causing less colloid P in the supematant.Ultimately,we believe that the findings can provide a new way to deeply interpret the geochemical cycling of P,even when considering other contaminants such as organic pollutants,heavy metal ions,and arsenate at the sediment/soil-water interface in the real environment.