[en] Zetapotential measurements were performed on HA, before and after reaction with Eu in the presence and absence of excess carbonate, under alkaline conditions. The aim was to establish whether complexation resulted in a loss or gain of negative charge. The magnitude of the negative zetapotential in the NaHCO₃ system decreased in direct contrast to the predicted increase in negative charge due to EuHA(CO₃)₂^-2 formation. Unexpectedly in the NaCl system, where mixed carbonate complexation was impossible, the same decrease in zetapotential was observed. The observations raise questions regarding the correctness of the reported stoichiometry and/or log β value of the mixed complex. (orig.)