The stability constant of the mixed ligand complexes and simple ligand complexes formed between Tb3+, Dy3+, Ho3+, Er3+ with L-Tyrosine and acetyl acetone were determined by potentiometric titration in aqueous solution (30:1°C, I = 0, 1). The complexes in the molar proportion 1:2 have form of LnTyr+ and LnAcAc2+, Ln(AcAc)2 2+ (Ln: Tb, Dy, Ho, Er). The best formation of complexes occurs in the range of pH from 6 to 8. The stability constant of complexes increases from Tb to Er. The complexes in the molar proportion 2 proportion have form of LnAcAcTyt, following 1:4:2 proportion have form of Ln(AcAc)2 Tyr. The best formation of complexes occurs in the range of pH from 7 to 9. The stability constant of complexes decreases from Tb to Er. The mixed ligand complexes turned out to be much stronger than the simple ligand complexes.