pH-Metric and UV-visible spectrophotometry studies were performed in aqueous solution at 25°;C and 0.1 mol dm m 3 ionic strength (NaClO 4 ) in order to determine the acidity and the complexing ability of five ( o -hydroxyphenyl) mono-methylenephosphonic acids (H 3 L) toward Ce(III), Pr(III), Nd(III), Sm(III), and Eu(III). The p K a values of the ligand, the hydrolysis constants of the cations, the number, the nature of their metallic complexes present in solution, their stability constants, and their individual electronic spectra as restored by computation over a broad acidity range have been determined by potentiometry and UV-visible spectrophotometry between 200 and 550 nm and are reported. The formation of 1:1 species partially protonated [MLH x ] and totally deprotonated [ML] as well as hydroxo species [ML(OH) x ] has been established with lanthanides (III). Selectivity of complexation in the lanthanides and the complexing ability of the ligands are discussed.