The unique properties of platinum-group elements (PGE) have led to a large range of applications for these metals. Consequently, multiple emission pathways have been investigated and a relation was firmly established between global demand for these metals and their subsequent release in the environment. It is therefore necessary to better understand the potential risks associated with PGE. In metal toxicity studies, it is often assumed that thermodynamic equilibrium in the exposure medium is reached rapidly. This however, may not be true for platinum (Pt) and palladium (Pd) due to their inherently slow kinetics. Indeed, the water exchange rate -as key indicator of reaction kinetics- for most transition metals is in the order of 10-7 to 10-10 s-1, while for Pt and Pd, the rate constants are orders of magnitude higher (~ 103 s-1). If the equilibrium period is long relative to the total exposure time, aquatic organisms may be mostly exposed to the complexes present at the start of the test rather than those expected to be present at equilibrium (or to a mixture of the two). In this project, we examined the accumulation of five Pt and Pd complexes (Pt(IV)Cl62-, Pt(II)Cl42-, Pt(II)(NH3)42+, Pd(II)Cl42- and Pd(II)(NH3)42+) by the unicellular green alga Pseudokirchneriella subcapitata and their subsequent toxicity after 96 h of exposure in USEPA medium. An investigation of the stability of the tested complexes was carried out using spectrophotometry measurements and kinetic calculations. Our results point at the rapid dissociation of Pd complexes leading to the formation of Pd(OH)2 in the case of Pd(II)Cl42- and the formation of a mixture of Pd(NH3)2+, Pd(OH)2 and Pd(NH3)22+ in the case of Pd(II)(NH3)42+. Similar Pd accumulation was measured for both complexes but Pd(NH3)42+ was slightly more toxic, maybe due to the presence of Pd(NH3)2+and Pd(NH3)22+. On the other hand, all three Pt complexes tested resulted in similar accumulation by the algae but only Pt(IV)Cl62- led to a significant growth inhibition after 96 h. Interestingly, Pt(II)(NH3)42+ was shown to be stable in test medium for at least 96 h whereas Pt(II)Cl42- was much less stable with indications of dissociation occurring within less than 24 h. This suggests that, initial speciation of Pt(II) complexes does not affect toxicity over the concentration range tested (0 - 150 µg/L). Furthermore, the surprising Pt(IV)Cl62- toxicity was potentially linked to free radical species formed during the reduction to Pt(II).