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Rare earth element (REE) concentrations were determined in circumneutral pH (7 less than or equal to pH less than or equal to 9) groundwaters from south-central Nevada and the Mojave Desert in eastern California, U.S.A. The inorganic speciation of the REEs in these water were evaluated primarily to assess the relative importance of carbonate (LnHCO(3)(2+), LnCO(3)(+) and Ln(CO3)(2)(-)) and phosphate (LnH(2)PO(4)(2+), LnHPO(4)(+), Ln(HPO4)(2)(-) and LnPO(4)(0)) complexes. Other REE complexes with sulfate, hydroxyl, chloride, and fluoride tons were also assessed, as was the significance of the free metal ion species (i.e., Ln(3+)). Our speciation calculations indicate that REE-carbonate complexes dominate and typically account for more than 99% of each REE in solution. Moreover, carbonate complexes (LnCO(3)(+)) were predicted to be the dominant species for the light REEs and dicarbonato complexes (Ln(CO3)(2)(-)) were predicted to increase in importance with increasing atomic number. All other complexes were predicted to be negligible. Furthermore, the combined percentage of all REE-phosphate complexes never accounted for more than 0.1% of the dissolved REEs, and generally much less than 0.1%. Phosphate complexes can not compete with carbonate complexes in these groundwaters because of the low Sigma PO43- concentrations (< 0.3-1.58 mu mol/kg), the much lower concentrations of the free phosphate ion (i.e., [PO43-](F) = 10(-9)-10(-6) mu mol/kg), due to ion pair formation with Ca2+ and Mg2+, and the much higher free carbonate ion concentrations ([CO32-](F) = 0.32-87 mu mol/kg).