A bicomponent initiation system consisting of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and the water soluble initiator potassium persulfate (KPS) was used to develop a robust and versatile semibatch emulsion polymerization process to obtain polystyrene (PS) latexes with solids contents of 5-40 wt %. A window of operating conditions was found that yielded high conversion (>95%) stable latexes and well controlled polymers, overcoming limitations found in previous attempts at developing similar processes using TEMPO. The critical parameters studied were surfactant concentration, monomer concentration in the nucleation step and the monomer feed rate in the semibatch step. Methyl acrylate (MA) was used in the nucleation step to improve the nitroxide efficiency (N-Eff). Latexes having molecular weight distribution (MWD) with dispersity (-D) lower than 1.5, average particle size (D-p) from approximate to 32 to approximate to 500 nm, nitroxide efficiencies N-Eff up to approximate to 1.0 and monomer conversions >90% were obtained in less than 12 h with solids contents up to 40 wt %. These results constitute a significant advance over prior efforts in TEMPO-mediated polymerization in aqueous dispersions. (C) 2015 Wiley Periodicals, Inc.