The mixed SDS/NP-40 (anionic/nonionic) surfactants were used to examine the generality of Smith-Ewart theory which was originally proposed for emulsion polymerization systems containing anionic surfactants. Our results are consistent with Smith-Ewart theory only when the wt% of NP-40 in the surfactant mixture ([NP-40]) is less than 30%. However, the reaction system deviates from Smith-Ewart theory dramatically when [NP-40] is greater than 50%. The steric stabilization effect provided by pure NP-40 is not strong enough to prohibit the interactive particles from flocculating with one another. On the other hand, the mixed surfactant system can greatly improve the latex stability via the synergetic effects provided by both the electrostatic and steric stabilization mechanisms and, thereby, retard the limited flocculation process. The mixed surfactant system SDS/NP-40 (20/80) is the best, because it results in the best reproducibility of the experiment and the greatest polymerization rate.