The mechanism of reversible-deactivation radical polymerization (RDRP) mediated by chloro-substituted amine-bis(phenolate)iron complexes was studied via a combination of Mossbauer, EPR, NMR, and online Vis/NIR spectroscopy. It was found that styrene polymerization operates via an atom-transfer radical polymerization (ATRP) mechanism when alkyl halide is present, whereas an interplay between ATRP and organometallic-mediated radical polymerization (OMRP) equilibria occurs during MMA polymerization. The preparation of the amine-bis(phenolate)iron(11) complex, [O2NN']Fe-II, allowed polymerizations to be performed under an OMRP-only regime free of alkyl halide that yields dispersities as low as 1.29 for MMA polymerization.