In this paper, the paramagnetic properties of a novel magnesium ketyl radical (compound 1), formed by reduction of benzophenone with a dimeric Mg(I) complex in the presence of dimethylaminopyridine, are described. Using CW EPR, ENDOR and special TRIPLE resonance, the spin distribution in the radical has been explored at variable temperatures (200-298 K). At 298 K, most of the unpaired spin is found to be confined to the (OCPh(2)(center dot)) fragment based on the hyperfine couplings (hfc s) of o-H = 8.30, m-H = 3.00 and p-H = 9.95 MHz. Smaller hfc s to (25) Mg (5.54MHz) and (14)N(DMAP) (0.90MHz) were also evidenced in the 298 K EPR spectrum, indicating some spin delocalisation onto the Mg(Nacnac)(DMAP) fragment. At lower temperatures, restricted rotations of the diphenyl rings create an inequivalent spin distribution in the two rings, with o(1)-H= 8.80, o(2)-H= 7.85, m-H = 3.00 and p-H = 10.00 MHz.