The mechanism of narrowly dispersed latex formation in a surfactant-free emulsion polymerization (SFEP) of styrene in acetone-water was studied by a combination of transmission electron microscopy (TEM) and dynamic light scattering (DLS). The critical nuclei were experimentally observed and the formation of narrowly dispersed PS latex is proved to be originated from competitive growth kinetics. Spherical nuclei were regenerated via a microphase inversion of PS oligomer in a 50% volume fraction acetone-water mixture at 70 degrees C. They followed a polydispersed log-normal distribution with (R) over bar (s) similar to 10.6 nm and delta similar to 0.298, and the smallest nucleus with R-s similar to 1.1 nm was similar to critical nuclei, with PS backbone (average chain length similar to 6-7) repeat units) inside and sulfate groups to stabilize it at oligomer chain ends. Note that the spherical nuclei are not necessarily narrowly dispersed. Competitive growth kinetics makes smaller nuclei grow much faster than large nuclei in the subsequent polymerization process, resulting in narrowly dispersed PS latex. Time resolved dynamic light scattering (DLS) was used to verify this. Two kinds of PS seed particles which had already grown to the dimension of < R-h > similar to 300 nm and similar to 400 nm were added, separately, into two parallel surfactant-free emulsion polymerization batches of styrene in acetone-water mixture at 70 degrees C, when the average hydrodynamic radius of PS latex was about similar to 20-30 nm. It was found that the size of similar to 300 or 400 nm seed particles almost did not change, but the small size PS latex grew rapidly. Narrowly dispersed PS latex was finally obtained in the SFEP system, which supports the competitive growth mechanism proposed by Vanderhoff and co-workers.