Magnetic properties and crystal structures of α-and β-2-hydro nitronyl nitroxide radicals (2-hydro-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide) were investigated. Exchange couplings in both phases were estimated from the parainagnetic susceptibility (1.8-300 K) and magnetization (up to 400 kOc) measurements. Assignment of the couplings to molecular packings has been carried out. Close spacing between the ONCNO moietics yields various magnitude of exchange couplings from 0 toca.-30 K. In α-phase crystal, short contacts between the central carbon atoms with negative spin density and the NO groups with positive spin density were observed. However, this packing is assigned to an antiferromagnetic exchange coupling,J/kB= −11 K (ℋ= −2JS1.S2). This case suggests that care must be taken when we refer to McConnell's proposal.