Single-electron transfer living radical polymerization (SET-LRP) has emerged as a reliable, robust, and straight forward tool for the construction of well-defined polymers. This manuscript reports on the influence of acidity on the SET-LRP process. The SET-LRP of methyl methacrylate initiated with tosyl chloride and catalyzed by Cu(0)/Me-6-TREN at 50 degrees C in protic solvent mixtures containing acetic acid is described. The polymerization kinetics was recorded in the presence of various amounts of acetic acid. Hereby it was found that the SET-LRP process tolerates considerable acidity. The SET-LRP of MMA comprising 1, 2.5, 10, and 25% acetic acid were well controlled. The reactions followed first-order kinetic in monomer consumption and the molecular weights increased linearly with conversion while retaining narrow molecular weight distribution. In all cases, complete monomer conversion was achieved. Acetic acid could be also used as a solvent for the SET-LRP of MMA. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4889-4893, 2010