Dynamic nuclear polarization with paramagnetic agents can enhance nuclear magnetic resonance signals by orders of magnitude. The mechanism of enhancement depends a.o. on the magnetic resonance properties of the agents used. Electron spin relaxation times at NMR relevant magnetic field/microwave frequencies, starting at 9 Tesla/275 GHz – still only 420 MHz NMR -, determine the efficiency as well. The Leiden 275 GHz spectrometer provides the high sensitivity and resolution required for such studies.