The modeling of the semibatch emulsion polymerization of styrene and its validation against data obtained from a reactor facility is presented. The model, which describes the growth of a monodisperse polystyrene seed as neat monomer is fed to the reactor, incorporates recent findings in radical diffusion and kinetics. The current controversy surrounding radical absorption into particles is handled by considering absorption via propagation, diffusion, and collision in the model. Simulation results including weight fraction polymer inside the particles and particle diameter are compared to data obtained from a custom-designed and built automated reactor control facility capable of on-line density and on-line particle diameter measurements. Good agreement between simulation results and experimental data are obtained for any of the three absorption mechanisms considered by varying only one adjustable parameter located in the absorption rate coefficient relation. A sensitivity analysis of the model to this adjustable parameter, using the program ODESSA, is also presented and shown to be an important tool in the validation procedure. Lastly, an analysis of the dynamics of the process shows the variety of phenomena that can be obtained in a semibatch reactor including regions that exhibit pseudosteady states, autoacceleration of the rate, and limiting conversion.