[en] Complex formation of all the series of rare earth with o-phenanthroline is studied. Using the method of pH-titration curves of formation of complexes are obtained, their stability constants being found. Reliability of the results, obtained using the method of pH-titration is reaffirmed spectrophotometrically. The data obtained are used to describe extraction equilibria in the lanthanoid-o-phenanthroline-caproic acid system