The effect of pH on Pb bioaccumulation by Chlorella kesslerii was studied. Both Pb uptake fluxes and Pb bound to membrane transport sites increased with an increase in pH from 4.0-5.0, were relatively stable in the pH interval 5.0-6.5, and increased again at pH 7.0 and 8.0. Protons affected Pb adsorption to the algal surface by competing directly for surface sites, by modifying the overall algal surface charge and by modifying the chemical speciation of Pb in solution. These results indicate a failure of the free-ion activity or biotic ligand models above pH 6.5, possibly due to the bioaccumulation of hydroxo or carbonato Pb complexes.