The partition of phosphorus between the organic fraction (PORG) and both the detrital (apatite rich) and non-detrital (adsorbed) fractions (PAP and PCDB) was studied in the particulate matter of the St. Lawrence Estuary and two of its tributaries. The adsorption of the PCDB fraction on particulate iron was examined. PORG makes up about one third of the total particulate P, which ranges betweeen 44 and 64 μg∙at./g. Everywhere in the estuary, and regardless of salinity, the concentration of PAP represents twice that of PCDB. The hydroxide fraction (FeCDB) is on the average 25 and 29% of the total particulate iron in May and September, respectively, but is much higher in the tributaries. The atom ratio (Fe/P)CDB of hydroxide Fe to adsorbed P fluctuates around 16 for May and 19 for September. Low values (13–16) are characteristic of the turbidity maximum, whereas higher ones are found in the deeper, more saline waters (16–26) and in the tributaries (17–26). Although any iron precipitation in the low-salinity turbid zone is masked by sediment resuspension, the low values of the ratio there suggest some P uptake on Fe-rich particles. Dissolved phosphate being conservative with respect to the salinity, desorption does not take place, or is only minor seaward of the turbidy maximum.