A single crystal X-ray diffraction (XRD) study of the radical cation salt (EDT-TTF) 3 [Fe(CN) 5 NO] has been carried out (a=6.623(3), b=10.487(2), c=15.951(1)Å, α=109.19(1), β=96.30(2), γ=99.15(3) o , V=1017.3(5)Å 3 , space group P1, Z=1). The crystals have a layered structure with the distinctive feature of the presence of alternating EDT-TTF dimers and monomers in the radical cation stacks. Both the structural features and the calculated HOMO...HOMO interaction energies suggest that the strong intradimer interaction is responsible for the charge separation and the semiconducting properties of the salt.