By method of potentiometric titration there were determined the constants of stability of complexes of neodymimum(III) with azoderivatives of pentane-2,4-dion – 3-(2-hydroxy-3-sulpho-5-nitrophenilhydrazo)pentane-2,4-dion (L₁), 3-(2-hydroxy-3-disulpho-5-nitrophenilhydrazo)pentane-2,4-dion (L₂), 3-(2-hydroxy-3-sulpho-5-chlorinephe-nilhydrazo)pentane-2,4-dion (L₃), 3-(2-hydroxy-4-nitrophenilhydrazo)pentane-2,4-dion (L₄), 3-(2-hydroxyphe-nilhydrazo)pentane-2,4-dion (L₅), which are changed in next order L₅>L₄>L₃>L₂>L_1.  The complex formation of neodymium(III) with L_1-3  has been spectrophotometrically investigated. It is found that with increasing negative inductive effects of functional groups, inserted into aromatically part of molecule, optimal conditions of complex formation shift to more acid medium and they do not affect on ratio: Nd:L=1:2. The influence of foreign ions and masking compounds on complex formation has been investigated. The technique of photometric determination of neodymimum(III) in rocks has been worked out