The geometries and energetics of the spin trapping reaction of the nitrone function have been determined using ab initio methods at the Hartree-Fock and second-order Moller-Plesset levels with the 6-31G(d) and 6-31G(d,p) basis sets. Radicals (H-., (CH3)-C-., (OH)-O-., and (OOH)-O-.), the prototype nitrone spin trap (H2C=NHO), and the resultant spin adducts were examined to ascertain the most probable site of radical addition, the minimum-energy geometries of the adducts, and the energy changes involved in the addition reaction. The calculations show that addition clearly favors the C-site of the nitrone, with the O-site being the second location when double spin adducts form; this supports experimental observations. Double spin adducts an energetically preferred over monoadducts. Addition at the C-site is highly exothermic, with no activation energy barrier for any of the radicals studied.