The termination processes of the tertiary carbomethoxyisopropyl radical, (COOMe)Me2C•, have been investigated by 1H NMR under a variety of conditions. The radical was generated by thermal activation of dimethyl 2,2′-azobis(2-methylpropionate) (V-601) and by bromine atom transfer from methyl 2-bromoisobutyrate (MBiB) to CuI/L activators (80 °C in CD3CN, CuI = [Cu(MeCN)4]+BF4–, L = tris[2-(dimethylamino)ethyl]amine (Me6TREN), tris(2-pyridylmethyl)amine (TPMA), 2,2′-bipyridine (bpy); 75 °C in C6D6; CuI = CuBr, L = Me6TREN, TPMA) in the presence of various additives: the CuI/L complex (for the V-601 decomposition), Cu0 (in C6D6), and/or proton/deuteron donors (CH3OH, CD3OD, Et3NH+BF4–). The investigation has allowed an assessment of the relative contributions of the CuI/L-catalyzed radical termination (CRT) and the reductive radical termination (RRT) to the spontaneous bimolecular radical terminations (RT) of the diffused radicals. The investigation also allowed reassessing the product distribution for the RT of the diffused and caged radicals obtained from the thermal activation of V-601. Both CRT and RRT were found to be minor but not negligible when the radical is slowly produced from V-601. The more rapid radical generation by atom transfer from MBiB maintains RT and CRT and decreases the impact of RRT. The catalytic activity in CRT decreases in the order CuI/TPMA > CuI/Me6TREN and is less prominent in C6D6 than in CD3CN, whereas the RRT activity increases in the order CuI/TPMA < CuI/Me6TREN and is less prominent in CD3CN. DFT calculations validate the principle that CRT occurs via the formation of a latent organometallic (COOMe)Me2C–CuII/L intermediate.