The radical anion of duroquinone (2,3,5,6-tetramethyl-1,4-benzoquinone) was used as a model compound to describe the low-frequency (LF) ENDOR lines observed in many radical systems. These LF lines are shown to arise from two-photon transitions, and they occur because of the rapid cross-relaxation rates and dynamical processes of the system. The LF lines occur when the radio frequency (RF) and microwave frequency match the energy of an adjacent EPR transition. In practical work, LF ENDOR lines disturb the determination of nuclei such as H-2, C-11, Na, and K, whose Larmor frequencies are in that region. Under certain conditions, two-photon transitions were also observed in the normal proton coupling region. A method for distinguishing normal ENDOR lines from two-photon transitions is given. The method is based on the different shape of an ENDOR-induced EPR spectrum when it is measured from two-photon-transition ENDOR lines.