The competitive binding of rare earth elements (REE) to humic acid (HA) and carbonates was studied experimentally at various pH and alkalinity values by combining ultrafiltration and inductively coupled plasma mass spectrometry techniques. The results show that the REE species occur as binary humate or carbonate complexes but not as ternary R EE-carbonate-humate as previously proposed. The results also reveal the strong pH and alkalinity dependence of the competition as well as the existence of a systematic fractionation across the REE series, Specifically, carbonate complexation is at a maximum at pH 10 and increase with increasing alkalinity and with the atomic number of the REE (LuCO3 >> LaCO3). Modeling of the data using Model VI and recently published stability constants for complexation of REE by humic acid well reproduced the experimental data, confirming the ability of Model VI to accurately determine REE speciation in natural waters. This modeling also confirms the reliability of recently published stability constants. This work shed more light not only on the competition between carbonates and HA for REE complexation but also on the reliability of WHAM 6 and Model VI for calculating the speciation of REE with organic matter in alkaline organic-rich water. (c) 2006 Elsevier Inc. All rights reserved.