A speciation analysis for Fe-mesohemin-(MPEG(500))(2)-mediated reversible-deactivation radical polymerization (RDRP) in aqueous solution was carried out by a combination of visible (vis) and Fe-57 Mossbauer spectroscopy. The results were used within kinetic studies of ATRP and OMRP reactions via highly time-resolved EPR spectroscopy. ATRP control was effective with the rate coefficient for deactivation clearly exceeding the one for formation of organometallic species. Deactivation rate coefficients increase by more than 1 order of magnitude in passing from polymerization in 30 to 90 wt % H2O. Media with water contents of and above 70 wt % are well suited for controlled ATRP. The Fe-mesohemin catalyst provides an exceptionally high ATRP equilibrium constant even at ambient temperature, which approaches the one of highly active Cu catalysts.