Halide exchange during atom transfer radical polymerization (ATRP) using mixed halide initiation systems, R-X/Cu-Y (X, Y = Cl or Br), was investigated. Model studies of mixed halide initiation systems (i.e., R-X/Cu-Y, X not equal t Y) demonstrated that exchange occurs rapidly at 90 degrees C, and there is a clear preference for alkyl chlorides to be formed in over alkyl bromides. This was attributed mainly to the carbon-chlorine bond being stronger than the corresponding carbon-bromine bond. This implies that, in ATRP with a mixed halide initiator/catalyst system, the bulk of the polymer chain ends are terminated by chlorine if[CuCl](0) greater than or equal to [RBr](0). Examples of using this information to improve the control in ATRP of methyl methacrylate (MMA) are presented. It was shown that, when benzyl halides were used as the initiator in the ATRP of MMA, the rate of initiation was increased relative to the rate of propagation, thus providing better control by using the benzyl bromide/copper chloride mixed halide system. Better molecular weight control at high conversions of monomer to polymer was observed when using ethyl 2-bromoisobutyrate and copper chloride as initiator/catalyst in comparison to using ethyl 2-bromoisobutyrate/copper bromide, indicating that side reactions are less significant in the former.