Recent experimental determinations of the solubility products of common rare earth minerals Such as monazite and xenotime and stability constants for chloride, sulfate, carbonate and hydroxide complexes provide a basis to model quantitatively the solubility, and therefore the mobility, of rare earth elements (REE) at near surface conditions. Data on the mobility of REE and stabilities of REE complexes at near-neutral conditions are of importance to safe nuclear waste disposal, and environmental monitoring. The aim of this study is to understand REE speciation and Solubility of a given REE in natural environments. In this study, a series of formation constants for La aqueous complexes are recommended by using the specific interaction theory (SIT) for extrapolation to infinite dilution. Then, a thermodynamic model has been employed for calculation of the solubility and speciation of La in soil solutions reacted with the La end-member of mineral monazite (LaPO4), and other La-bearing solid phases including amorphous lanthanum hydroxide (La(OH)3, am) and different La carbonates, as a function of various inorganic and organic ligand concentrations. Calculations were carried out at near-neutral pH (pH 5.5-8.5) and 25 degrees C at atmospheric CO2 partial pressure. The model takes account of the species : La3+, LACl(2+), LaCl2+, LaCl30, LaCl4-, LaSO4+, La(SO4)(2)(-), LaCO3+, La(CO3)(2)(-), LaHCO32+, La(OH)(2+), LaOx(+), La(Ox)(2)(-), LaAc2+ and La(Ac)(2)(-) (where Ox(2-) = oxalate and Ac- = acetate).