A PHREEQC model was written to investigate complexation of dREEs with components of the dissolved OM pool. Based on the data measured during a mesocosm experiment (maybe link the original dataset) the model simulated chemical speciation of the dREEs in the mesocosms. A new databank was created that includes stability constants for complexes of dREEs with the main inorganic ligands (Cl⁻, SO₄⁻, OH⁻, CO₃⁻) as well as with the strong organic ligand desferrioxamine B (DFOB) after Christenson and Schijf (2011). The model outcome includes concentrations of inorganic and organic dREE complexes as well as abundances as free ions (REE^3+) and total dREE concentrations. Additionally, we calculated the proportions of the different complexes to the total dREE pool. We used two different approaches for the PHREEQC simulations that followed Schijf et al. (2015; doi:10.1016/j.marchem.2015.06.010) and were characterized by the concentration of the strong organic ligand and the resulting proportion of organic complexes to the dREE pool. The 'High-DOC' approach results in a maximal proportion of organic REE-DOC complexes of 40%, the 'Low-DOC' approach results in maximum of 10% organic complexes.To keep an eye on variations in carbonate complexes, total alkalinity (TA) was monitored as well. TA was sampled daily, for the analysis we used a multiscan GO microplate spectrophotometer (Thermo Scientific) and followed the method described by Sarazin et al. (1999; doi:10.1016/S0043-1354(98)00168-7).