Bimolecular termination in nitroxide-mediated radical polymerization in miniemulsion has been investigated through the heating of a polystyrene-2,2,6,6-tetramethylpiperidinyl-l-oxy macroinitiator and its 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy analogue in an aqueous toluene dispersion with sodium dodecyl benzenesulfonate as a surfactant at 125 degrees C. The level of bimolecular termination by combination, evaluated from the high-molecular-weight shoulder, was higher in miniemulsion than in solution and increased with decreasing particle size. Quantitative analysis revealed that these results cannot be rationalized solely by nitroxide partitioning to the aqueous phase. The results are explained by an interface effect, by which nitroxide is adsorbed or located at the aqueous-organic interface. (c) 2007 Wiley Periodicals, Inc.