The study reported here was carried out in the macrotidal estuary of Penzé (Brittany, Western Channel, France). Ten field stations along the freshwater–seawater mixing zone were sampled each month in order to examine the speciation of dissolved and particulate cadmium. Different biogeochemical parameters (suspended particulate matter, Chlorophyll-a, pH and dissolved organic carbon) were also measured. Levels of total dissolved and total particulate metal ranged from 0.06 to 0.46 nM and from 0.4 to 2.4 μg g−1, respectively. Our data showed a non-conservative behaviour for cadmium as a result of desorption from particles. The rise in total dissolved cadmium in the salinity range 0–20 was about 0.2 nM. Incidentally, a decrease of total particulate concentrations of about 0.5 μg g−1, which corresponds approximately to 0.2 nM, was observed. The study of the various cadmium species led us to identify desorption of cadmium occurred from the iron/manganese oxide-associated fraction. This desorption process leads to the formation of labile complexes, i.e. chlorocomplexes in spring and summer. However, in winter and autumn, a large part of the dissolved fraction was constituted by organic complexes. These complexes can be formed from suspended particles desorption or could be a released from sediment. Analysis of flux indicated that most of the metal inputs to the estuary (usually more than 80%) corresponded to organic cadmium. Spring and summer outputs were mainly constituted of chlorocomplexes whereas cadmium organic complexes were predominant in winter and autumn output fluxes.