A flow injection (FI) on-line sorption system, for the preconcentration and separation in a knotted reactor (KR), is coupled with inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) for the fast, selective and sensitive determination of ultra-trace concentrations of rare earth elements (REEs) in environmental waters. The on-line preconcentration and separation of the REEs is achieved by sorption of the REE complexes formed in the pH range 3.7–4.6 on the inner walls of a (300 cm × 0.5 mm) PTFE KR pre-coated with the chelating reagent 1-phenyl-3-methyl-4-benzoylpyrazol-5-one (PMBP) at a pH of 9.6. The analytes are eluted and transported to the ICP-TOFMS system with 2% (v/v) HNO₃ containing 0.5 µg l⁻¹ of Rh and In as internal standards, using ultrasonic nebulisation. The REEs are effectively preconcentrated and separated from the major matrix constituents—alkali and alkaline earth elements. Of particular significance is their separation from barium, which produces considerable isobaric interferences on some of the analytes. Using a preconcentration time of 30 s and a sample flow rate of 4.4 ml min⁻¹, enhancement factors of 15 to 22 for the different REES are achieved. The detection limits (3σ) range from 3 to 40 pg l⁻¹. The precision for 9 replicate determinations of 0.1 µg l⁻¹ of each analyte is better than 5% (RSD). The accuracy of the method is demonstrated by the analysis of environmental waters and by recovery measurements on spiked samples.