Particle formation and coagulation in the seeded semibatch emulsion polymerization of butyl acrylate were studied under monomer-starved conditions. To investigate the importance of the kinetics of the water phase in the nucleation process, the monomer feed rate was used as a variable to alter the monomer concentration in the aqueous phase. The emulsifier concentration in the feed was employed to alter the particle stability. Particle formation and coagulation were discussed in terms of critical surface coverage ratios. Particle coagulation occurred if the particle surface coverage dropped below cr1 ฯญ 0.25 ฯฎ 0.05. The secondary nucleation occurred above a critical surface coverage of cr2 ฯญ 0.55 ฯฎ 0.05. The number of particles remained approximately constant if the particle surface coverage was within cr1 ฯญ 0.25 ฯฝ ฯฝ cr2 ฯญ 0.55. This surface coverage band is equivalent to the surface tension band of 42.50 ฯฎ 5.0 dyne/cm that is required to avoid particle formation and coagulation in the course of polymerization. The kinetics of the water phase was shown to play an important role during homogeneous and micellar nucleations. For any fixed emulsifier concentration in the feed and above cr2 , the number of secondary particles increased with monomer con- centration in the aqueous phase. Moreover, the presence of micelles in the reaction vessel is not the only perquisite for micellar nucleation to occur, a sufficient amount of monomer should be present in the aqueous phase to enhance the radical capture by partially monomer-swollen micelles. The rate of polymerization increased with the surfactant concentration in the aqueous phase.