The very early stages of the emulsifier-free emulsion polymerization of styrene were investigated by on-line monitoring of the optical transmission and the conductivity of the reaction mixture. Very careful degassing of the reaction mixture is crucial to achieve high reproducibility. The higher the residual gas concentration the poorer the reproducibility no matter if the gas is air or nitrogen. The results lead to the conclusions that for particle nucleation the rate of initiation in the water phase is very important and that nucleation occurs via cluster formation of water-born oligomers. With the experimental techniques employed, it was possible to investigate the conversion range from 0.47% to 1.8%. However, it was not possible to detect a particle number maximum. Instead of a maximum, a decrease in the particle number was observed followed by a leveling and another increase, indicating another nucleation step.