For the selective separation of rare earth(III) ions (RE(III)), flotation spectrophotometric behaviors of these ions were investigated using alizarin both in the absence and in the presence of 1,10-phenanthroline (phen). The synergic effect of phen during the formation of stable ternary complexes with RE(III) ions, which influences the flotation process, was evaluated spectrophotometrically. After the pH of the media was varied and the other parameters were kept constant, the flotation process was followed with separation and dissolution of the complex in N,N-dimethylformamide (DMF) for measuring the absorbance. It was shown that after the pH was raised from 5, the synergic effect was observed for a number of light RE(III) ions followed for the others as the pH was increased to higher values. These observations, on one hand, are related to the others in terms of the characteristic properties of alizarin, phen, and their interactions, moreover to the type of RE(III) ions, so that the revealed differences between the properties of RE(III) ions can be realized for mutual separation of these ions.