The kinetics of the ferric ethylenediaminetetraacetate (EDTA)–diisopropylbenzene hydroperoxide (DIBHP)-sodium formaldehyde sulfoxylate (SFS) redox system were determined. The rate constant is a function of the ionic strength μ. A plot of log10 (rate constant), for the rate-limiting second-order iron—SFS reaction, vs.μ1/2 has a slope of 1.3 with a limiting rate constant of 295 ± 15 L. mol−1. min−1 at μ = 0. The slope of this plot decreases to essentially zero for μ > 0.06M. Incorporation of these results into the Smith-Ewart model for emulsion polymerization correctly predicts the dependence of latex particle size on Fe and DIBHP concentration, but a much lower dependence on SFS concentration is observed.