Rare earth elements (REEs) have been shown to be efficient tracers of the functional sites and/or complexes formed on humic molecules. In the present study, we test the potential of REEs to be used as tracers of the sources of humic substances (HSs). Three types of organic matter (OM) of terrestrial and microbiological origin were tested. The experiments of REEs binding to the HSs were combined with size-fractionation experiments. The REE patterns were the most fractionated in the <10 kDa fraction. For Leonardite humic acid (LHA) and Aldrich humic acid (AHA), the REE patterns were consistent with the REEs binding to strong but low density sites for a low REE/C loading. By contrast, for Pony Lake fulvic acid (PLFA), the REE pattern was similar to the REE pattern developed onto a bacteria cell surface and was attributed to the REEs binding to phosphate surface sites. Fluorescence and elemental analysis of PLFA showed that the <10 kDa fraction was the fraction with the stronger microbiological character, which suggested the REEs were probably bound to PLFA through REE-phosphate complexes. Such results therefore provide a new possibility for the use of REEs to assess an OM source without the need to perform numerous or complex analytical methodologies.