The investigated complexation of rare-earth metals (REM)  – La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu with 3-(2-hydroxy-3-sulfo-5-nitrophenylhydrazo)pentane-2,4-dione (L) by spectrophotometric, conductometric and potentiometric titration methods. The constant stability of complexes REM with L changes in the following sequence: Lu>Yb>Tm>Er>Ho>Dy>Tb>Gd>Eu>Sm>Nd>Ce>La. It is established that with decrease of ions radius REM  stability constant increases and molar absorption and environment of the REM do not influence on comparison REM:L=1:2. The effect of foreign ions and masking agents on the complexation is studied. A spectrophotometric procedure was developed for determining of REM in industrial waste water