Carbon based paramagnetic materials are frequently used for EPR oximetry, especially in vivo, but the EPR spectra of these materials often show more than one paramagnetic center and/or relatively low signal intensity. To determine whether the components of carbon based materials could be separated and enriched in the active component, we used density gradient centrifugation to separate the materials into several fractions. We studied two types of coals, gloxy and Pocahontas, and found these materials to have large density distribution. The separated density fractions had very different EPR spectra and intensities. The active component from the coal material displays a more homogeneous EPR signal, and significantly increased EPR signal intensity. In similar studies with India ink, only slight changes were observed in attempts to separate the material. These results with coals may be very useful in the development of better probes for EPR oximetry.