Sodium trimetaphosphate was investigated as a new complexing agent for the cation-exchange separation of rare earths. Cation-exchange equilibrium distribution coefficients with Dowex 50 W-X8, 200-400 mesh, a sulfonated polystyrene resin, are presented for 15 rare earth elements and thorium in different concentration ranges between 0.01 and 0.3 M sodium trimetaphosphate. The distribution coefficients were determined at a total amount of cation to total resin capacity ratio q = 0.4. By considering these distribution coefficients, a good separation of all lanthanides were achieved by using concentration gradient elution with sodium trimetaphosphate at room temperature. Sodium trimetaphosphate has the following advantages over alpha-hydroxyisobutyrate as an eluting agent: The elution peaks are narrow and nearly symmetrical, the tailing effects are very small and there is no overlapping, the separation time is shorter, and the eluent concentration is smaller than those of other known methods. Dy and Y are well separated. This separation has been successfully applied to different rare earth mixtures for determination, purification, and preparative purposes. For example, it was applied to the 5 and 250 mg rare earth oxides obtained from monazite. All rare earths and thorium were separated in 23.5 minutes in small-scale separation by concentration gradient elution with sodium trimetaphosphate. Qualitative and quantitative determinations of eluted ions were performed by spectrofluorimetry.