A method is described to measure rare-earth elements (REE) in natural water samples. An iron hydroxide precipitation followed by ion-exchange chromatography is used to concentrate and separate the REE from other matrix components. Additionally, the rare-earth elements themselves ar separated into the fractions using an organic acid with a second cation column. Instrumental detection is by isotope dilution mass spectrometry (IDMS) using either thermal ionization mass spectrometry (TIMS) or inductively coupled plasma-mass spectrometry (ICP-MS). Excellent precision is shown by many duplicate and replicate analyses. Blanks are 1-2% or less of samples.