A comparison of the results of two models with different levels of complexity has shown that errors can be made in the calculation of the polydispersity ratio of a polymer, produced in emulsion in the presence of termination by combination, if compartmentalization is not taken into account by properly considering the distribution of the lengths of the pairs of chains belonging to the same particle. These errors have been shown to be significant for values of the average number of free radicals per particle between about 0.4 and 2, which are typical of many emulsion systems. Accounting correctly for compartmentalization becomes crucial when the termination by combination mechanism is responsible for gelation, as in the case where branching through chain transfer to polymer is present in the system. The nature of the effects of compartmentalization on molecular weight has been analyzed both from a physical and a mathematical point of view. The importance of using a detailed model has been discussed for three polymerization systems : styrene, methyl methacrylate, and vinyl acetate.