We report here for the first time a high-resolution study of the colloidal cadmium speciation with seasonal variations in estuarine waters. Over this 1-year study, seven colloidal fractions below 0.45 pm were analysed at 6 stations along the salinity gradient of the Penze estuary, with the aim to provide some insights into the mechanisms that control Cd concentrations and behaviour under estuarine mixing. A classic and strong positive Cd anomaly occurred at mid-estuary throughout the seasonal cycle. This anomaly corresponded essentially to an input of low molecular weight (LMW) compounds, i.e. in the size range 5-50 kDa, arguing that besides the usual hypothesis of Cd complexation with chloride, it is also of fundamental importance to consider the role of colloidal organic species in favouring Cd mobilisation. The colloidal repartition of Cd being notably different to that of humic substances, the ligands involved in Cd complexation are more probably of allochthonous origin. Over the annual cycle, total dissolved Cd (<0.45 mu m) concentrations diminished considerably in spring and summer over the whole system, while an increasing of Cd in particles at high salinities was taking place at the same periods. The effects of biological uptake, of the associated pH increase of the waters and of the nature of organic ligands on (1) Cd desorption mechanism and (2) on the resulting Cd speciation are discussed.