Size control is one of the most important tasks in colloid synthesis. In the present work, the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) were used to precisely control the size of poly-N-isopropylmethacrylamide [poly-(NIPMAM)I microgels prepared by precipitation polymerization. For each surfactant the concentration was varied over a broad range below the critical micelle concentration (cmc). The resulting particle size, size distribution, and shape were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Photon correlation spectroscopy (PCS) was employed to analyze the volume phase transition behavior of the microgels. We show that for both surfactants a decrease in particle size is achieved for increasing surfactant concentrations while the swelling properties of the microgels are nearly unaffected. (C) 2013 Elsevier Ltd. All rights reserved.