In this research, poly(3,4-ethylenedioxythiophene) (PEDOT) latex nanoparticles with good colloidal stability were prepared by emulsion polymerization and the conversions of EDOT were determined. Two kinds of oxidants, Iron(III) p-toluenesulfonate Fe(OTs)3 and hydrogen peroxide H2O2, were introduced to decrease the use of iron salt and therefore reduce the particle coagulation. The ferrous ions (Fe2+) produced during the polymerization would be re-oxidized back to the reactive ferric ions (Fe3+) with the help of H2O2. This cyclic oxidation–reduction process resulted in the sustained regeneration of Fe3+ ions and led to a higher conversion. A dark blue PEDOT latex with long-term dispersion stability was obtained when Fe(OTs)3 and H2O2 were added in sequence. The results obtained from dynamic light scattering and TEM measurements showed that the sizes of nanoparticles were around 100 nm. To determine the conversion of EDOT, two methods (gravimetric analysis and UV–visible method) were used and compared. For the first time, the UV–visible method was established to quantitatively determine the conversion of EDOT monomer. From the measurement, the conversion of EDOT in this system was determined as 74–75%. The PEDOT film prepared by drying the latex solution had conductivity up to 6.3 S/cm.