The complexation in ternary systems Ln-L-A (Ln—lanthanide ions Ln (III) ; L—acrylic, methacrylic, maleic acids ; A—acetylacetone) have been investigated by pH-potentiometric titration technique and by NMR spectra lineshape analysis on the base of quantum-mechanical formalism of spin density matrix. Stability constants, NMR spectra, paramagnetic shifts, relaxation times and lifetimes of mixed-ligand complexes were determined. General conformities of the complexation according to the lanthanide row were revealed. The dependences of complex compounds stability on the lanthanide ionic radius are explained by joint influence of electrostatic and steric effects.