In emulsion polymerization processes it is important to control the surfactant level to avoid secondary nucleation (too much surfactant) and coagulation (too little), Seeded emulsion polymerizations are especially sensitive since the buffering effect of micellar soap is absent and the surfactant concentration is generally low, Surface tension measurements were employed to monitor the instantaneous free surfactant concentration, A previously developed Langmuir site adsorption model was employed to relate surface tension to surfactant concentration from first principles, This model describes the partition of added sodium dodecylbenzenesulfonate (SDBS) and mobile in situ surfactant between the aqueous phase and the particle surface in the presence of anchored sulfate groups, High solids contents in typical synthetic reactions result in significant contributions from the in situ surfactant and anchored groups, In a model MMA/BA latex, the agreement between observed and calculated surface tension was similar to 1 dyn/cm, enabling design control of starve fed emulsion polymerization, When the surface tension was maintained at 45-57 dyn/cm, the process successfully generated 500 nm to 3 mu m monodisperse particles.