Laser flash photolysis and kinetic competition product studies have demonstrated that in isooctane at ambient temperatures the rate constant for coupling of carbon-centered radicals with persistent nitroxides, k T , depends upon the degree of steric hindrance to coupling and upon the extent of resonance stabilization of the carbon radical. Sterically induced reductions in the magnitude of k T are observed for changes in both the structure of the nitroxide and the structure of the carbon radical. Thus, for any particular carbon radical k T is largest for the Bredt's rule protected nitroxides, 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) and nortropane-N-oxyl, while for the usual' di-tert-alkyl nitroxides k T decreases along the series, 1,1,3,3-tetramethylisoidoline-2-oxyl ≥2,2,5,5-tetramethylpiperidin-1-oxyl (Tempo)>di-tert-butyl nitroxide, i.e.,k T decreases on going from a five-membered ring to a six-membered ring to a noncyclic structure