The rate of ATRP deactivation was measured for the amine-bis(phenolate)iron-mediated polymerization of 2-ethylhexyl methacrylate (EHMA). EHMA radical concentration is monitored by highly time-resolved EPR spectroscopy after pulsed-laser-induced radical production. In addition, the concentration of the iron(III) complex was measured via EPR. The SP-PLP-EPR method allows for the analysis of ATRP deactivation rate without interference by potential organometallic reactions. Toward higher temperature, the ratio of deactivation to propagation rate increases, which is beneficial for ATRP control.