X-irradiation of single crystals of 5-chlorodeoxyuridine (ClUdR) and 5-bromodeoxyuridine (BUdR) at 300 K produces 3 different free-radical species which are isostructural in the 2-crystal systems. The .alpha.-halogen type R.ovrhdot.CHal(CH2)R'' was described previously. Of the remaining free radicals, one was analyzed by ESR-ENDOR [electron-nuclear double resonance] techniques. It exhibits a g-tensor with elements 2.0007, 2.0042 and 2.0065 and 3 hyperfine interactions: (4.6 G, 6.6 G, 4.3 G), (-4.8 G, -0.1 G, -5.1 G) and (2.0 G, 0.7 G, 5.6 G). These spectral parameters are assigned to a free-radical structure 0 = CH(.ovrhdot.C-OH)CHRR'' at the O5''C5''C4''C3'' site of the deoxyribose group resulting from ring opening subsequent to fragmentation of a precursor alkoxy radical at the O5'' locus. The 3rd radical species is inaccessible to ESR studies but gave rise to a distinct .alpha.-protein ENDOR-resonance line from which, as a tentative assignment, enolization of the C4-pyridine base-located carbonyl function can be derived. These structures are discussed in relation to the radiosensitizing properties of 5-halouracils when replacing thymine residues in DNA.