The utility of the growth mechanism to influence relative particle growth is investigated in a semi-batch reactor. A dynamic competitive growth model is developed and used to simulate the growth of two monodisperse polystyrene particle populations !bidisperse system) at 50 degrees C. Validation of the model with on-line density and on-line particle diameter measurements demonstrates that radical absorption into particles is more likely to occur by a collision mechanism than either by a propagation or diffusion mechanism. Experimental and simulation results show that relative narrowing of the bidisperse system increases as the weight fraction polymer inside the particles (W-p) decreases. It is concluded that the ability to alter W-p in a semi-batch reactor makes this mode of operation attractive for particle size distribution control.