Miniemulsion copolymerizations of methyl methacrylate and n-butyl acrylate initiated with water-soluble (hydrogen peroxide, HPO), partially water-soluble (tert-butyl peroxide, TBHP), and oil-soluble (tert-butyl peroctoate, TBPO) initiators were carried out to study the grafting of poly(vinyl alcohol) (PVA) at the oil/water interface. The amounts of grafted PVA produced in miniemulsion polymerizations initiated with TBHP and TBPO were substantially less than those produced in the corresponding seeded emulsion polymerizations. Differences in the internal viscosities at the respective interfaces were proposed to explain these results. The amount of grafted PVA was determined as a function of the monomer to polymer (M/P) ratio in seeded emulsion polymerizations initiated with TBHP and was found to decrease with increasing M/P ratio. The grafting was followed as a function of conversion in miniemulsion polymerizations initiated with TBPO and where a continuous addition of SFS (sodium formaldehyde sulfoxylate) solution was utilized; the grafting was found to occur mainly in the later stages of the polymerization. Both results strongly support the idea that the different internal viscosities at the interface might affect the termination reactions between primary radicals and PVA macroradicals, thereby resulting in different amounts of grafting. The degree of hydrolysis (DH) of the serum PVA after the miniemulsion polymerizations was determined using FTIR and saponification. On the basis of the results, it was found that aqueous phase and interface grafting occurred in the HPO system; however, interface grafting dominated the TBHP system. In addition, the grafted PVA in the TBHP system was still water-soluble, implying that the grafted chains were short, owing to rapid termination reactions at the interface presumably.