The atom transfer radical polymerization (ATRP) of styrene in water/toluene mixtures was studied. A linear dependence of the molecular weight on conversion was observed, but the initiation efficiency decreased when the catalyst concentration increased. The variation of the amount of water in the system affected the control of the ATRP, indicating that the presence of the aqueous phase influenced the concentration of copper halides in the organic phase. The partitioning of copper halides resulted in almost complete migration of Cu-II into the aqueous phase, which assisted with catalyst removal after polymerization. For example, the amount of residual copper in the organic phase determined by inductively coupled plasma was less than 1 ppm when the polymerization mixture was exposed to air for 30 min. The ATRP of styrene in water/toluene mixtures occurred with the preservation of Br at the polymer chain end, as confirmed by successful block copolymerization.