Recent improvements of a 3 mm high-field/high-frequency CW EPR/ENDOR spectrometer [O. Burghaus, M. Rohrer, T. Götzinger, M. Plate, and K. Möbius, Meas. Sci. Technol.3, 765 (1992)] are presented, making it possible for ENDOR measurements to be performed with NMR radiofrequeney fields of more than 2.5 mT (rotating frame) over a frequency range of 40 MHz. Applications of this setup to plastosemiquinone anion radicals are described which serve as a model for the secondary electron acceptor in plant photosystem II. The orientation-selected high-field ENDOR spectra of the plastoquinone anion radicals, recorded at different EPR field positions, are analyzed with the aid of computer simulations. The determination of hyperfine interactions of the radical in a perdeuterated solvent is described. The experiments demonstrate the potential of high-field/high-frequency EPR/ENDOR in structure determination including anisotropic interactions with the environment.