Most commercial EPR spectrometers work at microwave frequencies below 40 GHz; practical considerations such as sample handling, sensitivity, and equipment cost favor X-band (910 GHz) spectrometers over S (2-4 GHz)and Q-band (3436 GHz) extensions, which are normally used only for special purposes. There are, however, many unsolved problems in molecular spectroscopy, such as lack of resolution in disordered samples, which require taking advantage of higher microwave frequencies (hmf) and, correspondingly, higher magnetic fields, &. The first advantage of hmf EPR is the improvement of spectral resolution due to the larger Zeeman interaction. It brings about a more precise determination of g tensors of frozen solutions or polycrystalline samples (1-4). In spectra of yor X-ray irradiated single crystals with different sites and small g-factor anisotropy a better separation of EPR lines becomes possible at hmf (5). An increasing field of investigations is mixed radicals in solution with small g-factor differences and anisotropic rotation of radicals in viscous solvents (3,4). In the slow-motion regime hmf EPR is distinguished by its increased spectral sensitivity to motional dynamics and important applications in this field are in progress (6). The second advantage of hmf EPR is the higher sensitivity. The two important factors that determine the sensitivity of an EPR spectrometer are the filling factor and the loaded Q of the resonator. Although sensitivity does not increase with a high power of B,, (the power ranging between f and g depending on experimental conditions (7)), there is a significant sensitivity improvement compared with X-band EPR in the investigation of small single crystals. Another advantage of hmf EPR spectrometers becomes apparent when investigating high-spin transition metal complexes with large zero-field splittings. Xor Q-band frequencies are often too low for measuring the zero-field splitting parameters D and E. Examples for such a situation occur for many compounds of biological interest (2). With these advantages in mind hmf EPR spectrometers have been constructed in a few laboratories, at 70 GHz (X = 4.3 mm) (8), at 94 GHz (X = 3.2 mm) (3, 9), at 150 GHz (X = 2 mm) (I, 4, 5) and, most recently, even at 250 GHz (X = 1.2 mm) (6). Additional advantages are envisaged when extending hmf EPR to hmf ENDQR spectroscopy. Due to the larger Zeeman interaction, selective saturation of EPR lines