X-irradiation of L-arginine hydrochloride monohydrate crystals at 66 K led to at least five radicals detectable with K-band Electron Paramagnetic Resonance (EPR), Electron-Nuclear DOuble Resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques. Radicals R1a and R1b were identified as carboxyl-centered radicals from one-electron reduction, (H(2)OOC*)CH (NH(3))(+)(CH(2))(3)NHC(NH(2))(2)(+) in two different conformations arising from the two geometrically distinct molecules of the asymmetric unit in the crystal. Density-Functional Theory (DFT) calculations on cluster models constructed separately for each molecule of the asymmetric unit support the assignments. Radical R2 was identified as the decarboxylation radical, *CH(NH(3))(+)(CH(2))(3)NHC (NH(2))(2)(+). Radical R3, with two proton couplings and one nitrogen coupling, was identified as a radical with the unpaired electron localized on the guanidyl group, (-)(OOC)CH(NH(3))(+)(CH(2))(3)NHC*(NH(2))(2)(+). R3 is a product of one-electron reduction different from radicals R1a and R1b. DFT calculations on a cluster model reproduced the experimental values very well and thus supported the assignment of R3. Geometry optimization indicated that the guanidyl group transformed from planar to pyramidal upon trapping the electron. Radical R4 was identified as a side chain dehydrogenation radical, (-)(OOC)CH(NH(3))(+)(CH(2))(2)*CH NHC(NH(2))(2)(+). It was not possible to collect sufficient data to identify radical R5, although it clearly exhibited hyperfine coupling to one nonexchangeable beta-proton.