The ATRP of methyl methacrylate (MMA) with the system of FeCl(2)(.)4H(2)O, N,N,N',N",N"-penta(methyl acylate)diethylenetriamine (MA(5)-DETA) and ethyl 2-bromoisobutyrate (EBiB) showed high activity and is not well-controlled. The addition of a certain amount of FeCl(3)(.)6H(2)O as the additive into the above system results in the decrease of the polymerization rate and the improvement of the controllability. When SnCl(2)(.)2H(2)O was added instead of FeCl(2)(.)4H(2)O, a novel catalyst system SnCl(2)(.)2H(2)O/FeCl(3)(.)6H(2)O/EBiB/MA(5)-DETA formed shows better controllability, higher catalytic activity and higher initiating efficiencies (more than 90%) in comparison with the system FeCl(2)(.)2H(2)O/FeCl(3)(.)6H(2)O/EBiB/MA(5)-DETA. The polydispersities of the polymers kept range from M-w/M-n = 1.18-1.26. The real active species in the new system is Fe(II) formed from the reaction of FeCl3.6H(2)O with the active radical species produced from the reaction of SnCl(2)(.)2H(2)O with EBiB. (C) 2003 Published by Elsevier Ltd.