The kinetic and mechanistic role of oil-soluble initiators in micro-and macro- (conventional) emulsion polymerization was discussed in comparison with that of water-soluble initiators. The rate of microemulsion polymerization of styrene initiated by oil-soluble azo-type initiators such as 2,2'-azobis(isobutyronitrile) was found to be about 1/3 of that initiated by water-soluble initiators such as potassium persulfate even with the same rate of radical production in both systems. The reason for this was discussed in terms of the location of initiator but is still uncertain. In the macroemulsion polymerization of styrene initiated by oil-soluble azo-type initiators, on the other hand, the fraction of initiator dissolved in the water phase was demonstrated to be responsible for particle formation, but which portion of initiator partitioned into the water phase or into the polymer particles mainly participates in particle growth is still equivocal, although this problem has been long discussed.