The synthesis of waterborne nanocomposites in semicontinuous emulsion polymerization was investigated using a theory-guided strategy with the aim of achieving the best balance among small particle size, low surfactant concentration, and sufficiently high solids content. It was found that both kinetic (monomer feeding rate, radical generation rate, and temperature) and colloidal (ionic strength and polymer hydrophilicity) aspects were critical in the process. Waterborne nanoparticles as small as 13 nm were obtained with a solids content/(surfactant/polymer) ratio higher than 7.