We have investigated the effect of added polystyrene-b-poly(ethylene oxide) (SO) copolymer on the stability of oil-in-oil (O/O) emulsions containing polystyrene (PS) and poly(ethylene glycol) (PEG) in chloroform (CHCl3) and directly visualized the location of SO in the emulsions by using dye-labeled SO (SO*) with confocal laser scanning microscopy (CLSM). The emulsion formed by PS/PEG/CHCl3 = 14/6/80 (wt %) consisted of a droplet phase of PS in CHCl3 and a continuous phase containing PEG in CHCl3. SO*s with various molecular weights (M-n,M-SO) and volume fractions of the PS block in SO (f(PS)) were prepared via living anionic polymerization and subsequent end-esterification. The effect of SO on the droplet size in the emulsions was investigated as a function of both M-n,M-SO and f(PS). Increasing M-n,M-SO and decreasing f(PS) were effective at reducing the droplet size down to less than 1 mu m, which is 100 times smaller than in the absence of SO. The location of SO*s in the O/O emulsions was further investigated by CLSM. We found that the location of SO*s changed from the droplet interior to the liquid-liquid interface and then to the continuous phase with decreasing f(PS). We discuss the possible mechanism in terms of the relation of SO* location to the droplet size.