ABSTRACT: The goal of this study was to investigate if transparent exopolymer particles (TEP), carbohydrates, surface-active substances (SAS), reduced sulfur species (RSS), or thio/amino groups contribute significantly to the complexing capacity of phytoplankton exudates for Cu (L_TOTCu), Cd (L_TOTCd), or Fe (L_TOTFe). Complexing capacities and apparent stability constants (K_app) were determined electrochemically for Cu and Cd in cultures of the marine diatoms Thalassiosira weissflogii and Skeletonema costatum, and in a culture of the coccolithophore Emiliana huxleyi. Furthermore, the complexing capacity with Fe, Cu and Cd of 4 marine polysaccharides (PS) (phytagel, carrageenan, laminarin and alginic acid) was investigated. As expected, more Cu than Cd was complexed in the 3 phytoplankton cultures and in the phytagel solution. Size fractionation of the phytagel solution suggests that the binding capacity for Cu was more significant in the particulate fraction (>0.7 µm), indicating that Cu was preferably trapped within pores and channels of large hydrogels. In contrast, Cd binding sites were predominantly found in the fraction <0.7 µm, suggesting binding to the outer surfaces of gel particles to be of greater importance for larger ions. The K_app of the Cd complexes were higher than those of Cu, indicating stronger binding of Cd ions than of Cu ions. Solutions of carrageenan, laminarin and alginic acid did not form complexes with either Cu or Cd, and Fe-binding properties could not be detected for any of the 4 polysaccharide solutions. Thio/amino groups of sulfur-rich ‘glutathione’ type ligands were found in all phytoplankton cultures and were presumably responsible for the complexation of Cu. No consistent relationship was observed between TEP, carbohydrate concentration, SAS or sulfur content, or with the complexing capacity, emphasizing the high degree of heterogeneity of substance classes responsible for metal binding.