The atom-transfer radical polymerization of methyl methacrylate (MMA) in bulk at 70 °C, using ethyl 2-bromo-phenylacetate as an initiator, is quite rapid and very well controlled when catalyzed by FeBr2 in the presence of small amounts of simple inorganic salts (chlorides, bromides, iodides, hydroxides, carbonates, bicarbonates, and sulfates) and compares favorably in all respects, including technical simplicity, with the previously reported protocols that make use of salts of large organic cations or neutral organic ligands as promoters. A detailed investigation of the KBr-promoted process, supported by density functional theory calculations, suggests that the active form of the catalyst is a 1:1 adduct, which is stabilized in solution by weak MMA coordination to give an ion-paired K+[FeIIBr3(MMA)]− species with a tetrahedral anion. The addition of 18-crown-6 provides a less soluble but more active catalyst. FeCl2 performs less efficiently and the addition of small amounts of water to the system does not significantly affect the polymerization rate, whereas larger quantities result in rate reduction. The quality of the control is further improved when using 0.1 equiv of the FeBr3 deactivator.