The influence of natural organic matter (NOM) on the speciation and solubility of europium (Eu) was studied under geochemical conditions representative for the Boom Clay. Different organic matter types were used, and analysis was performed both after 0.45 μm microfiltration and after 30,000MWCO ultrafiltration. Equilibrium was approached from undersaturation starting from synthesised EuOHCO3(s). In the absence of NOM, the Eu solution concentrations after 0.45 μm filtration exceeded the thermodynamic solubility of EuOHCO3(s) by several orders of magnitude, indicating the presence of inorganic Eu colloids. In the presence of NOM, the Eu solubility increased with increasing NOM concentration, but, surprisingly, was dependent on the operational size cut-off. This latter observation necessitates detailed knowledge concerning the pore size cut-off of Boom Clay under in situ conditions in order to use the correct Eu-NOM complexation constant and/or maximum solubility in transport calculations. All of the data sets were modelled using the Nagra/PSI database and organic complexation reactions between Eu3+ and NOM functional groups. Both a free ligand approach (with electrostatic correction) and the humic ion-binding model VI, which was for the first time successfully introduced into Phreeqc geochemical code, were tested and provided equally good fits to the data.