W-band EPR (λ ≈ 3 mm, ν ≈ 95 GHz) investigations of frozen solutions of various semiquinone anion radicals in vitro are presented. Among the quinones studied are ubiquinone-10 and the primary electron acceptors of photosystem I and II of plant photosynthesis, vitamin K, and plastoquinone-9, respectively. In addition, W-band spectra of the primary quinone Q and of the radical pair PQ Zn-substituted reaction centers of Rhodobacter sphaeroides R-26 are shown. The electron Zeeman interaction at resonant B fields in the W-band is increased by a factor of 10 as compared with conventional X-band, leading to almost completely resolved g-tensor components in all quinone radicals. By means of computer simulations of the EPR spectra of the immobilized radical ions, anisotropic g tensor components, anisotropic line width contributions, and, in some cases, hyperfine tensor information could be obtained. The data are partially interpreted by semiempirical MO methods including solvent effects. In all quinone radical anions studied, the measured g-tensor components are assigned to the molecular axes as follows: g > g > g, where x is along the C-O bond direction and z is perpendicular to the quinone plane.