Modified Smith-Ewart equations were used to quantitatively investigate compartmentalization effects in nitroxide-mediated radical polymerization in dispersed systems. Calculations were carried out for the specific system 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO)/styrene at 125 degrees C with thermal initiation included in the model, and accounting for compartmentalization of both propagating radicals and nitroxide. For particles with diameter (d) less than approximately 60 nm, a reduction in particle size led to lower rate of polymerization, higher degree of livingness, and a higher number of activation-deactivation cycles experienced per chain to grow to a given degree of polymerization. These effects have their origin in the pseudo first-order deactivation rate coefficient increasing in proportion to d(-3) with decreasing particle size. There were no significant effects of compartmentalization for particles with d > 110 nm. The results show that it is important to consider compartmentalization effects also on the deactivation reaction, and not only bimolecular termination.