The crystal structure of 5-nitrothiophene 3-carboxaldehyde was determined. X-irradiation of these crystals, at 77 K, leads to the formation of a radical pair (D=−875 MHz, E=49 MHz) which exhibits hyperfine interaction with two protons. By using the structural information of the crystal structure together with the ESR tensors, it is possible to show that this pair is composed of an allyl-type radical and of a σ radical produced by a C–H homolytic scission. This pair disappears at 155 K. At 290 K a new species is created; the ENDOR analysis shows that this new radical results from a radical addition at the carbon bearing the NO₂ group.