The mole percentage of dead chains (T-mol %) was quantified in a normal Cu-mediated atom transfer radical polymerization (ATRP) of methyl acrylate with tris(2-pyridylmethyl)amine and tris[2-(dimethylamino)ethyl]amine as the ligands in acetonitrile. The value for T-mol % significantly exceeded the values predicted for conventional termination between two radicals. The main reason for the additional loss of chain-end functionality was identified as a Cu-I-induced catalytic radical termination (CRT). The mechanism proposed for this CRT involves the formation of highly active R-Cu-II/L and/or H-Cu-II/L intermediates which react with radicals to form dead chains and regenerate the Cu-I activator. The contribution of CRT to the loss of terminal functionality can be significantly decreased by reducing [Cu-I] to <1 mM, according to recently developed new ATRP processes.