Resonant absorption of electromagnetic radiation by paramagnetic particles in a constant magnetic field has found use in EPR spectroscopy, which detects paramagnetic particles and describes their interactions with neighbouring atoms, molecules, etc.

In a static magnetic field, the energy level splits (the Zeeman effect, see nuclear magnetic resonance). The energy splitting value and, thus, the resonant frequency are controlled by the electronic environment and the nature of intra- and intermolecular interactions. EPR is used to study systems with a non-zero electron spin magnetic moment, i.e., with one or more unpaired electrons (paramagnetic particles). Paramagnetic particles can be atoms and molecules (e.g., nitrogen and hydrogen atoms, NO molecules), free radicals (CH₃), point defects in the solids, transition metal ions (including members of the cluster compounds). For example, EPR has described the structure of (M³⁺@C82^3-, where M=La, Lu, Er, Y, etc.) endohedral fullerenes, with many of them being radicals.