A series of well-defined diblock, triblock, and star-block copolymers composed of polystyrene and poly(acrylic acid) were synthesized by controlled/living radical polymerization and used as stabilizers in emulsion polymerization under alkaline conditions. The structure of the copolymers, the size of the blocks, and the composition were varied and their efficiency as stabilizers was correlated with their structural characteristics. The block length was varied from 10 to 30 units for the polystyrene block and from 13 to 266 units for the poly(acrylic acid) block. The copolymers appeared to be efficient stabilizers down to a block copolymer-to-monomer ratio of less than 0.5 wt %. From the comparison of the effect of the different structures and compositions, it was shown that the diblock copolymers were particularly efficient and that the optimal composition was about 10 styrene units and a maximum of 50 acrylic acid units. The triblock and star-block copolymers with external hydrophilic blocks did not behave much differently than diblock copolymers. In contrast, for the triblock copolymers with an internal hydrophilic segment, the efficiency strongly depended on the respective length of both blocks. The evolution of the number of latex particles, N-p, with the concentration of surfactant was also studied and N-p was shown to be proportional to [surfactant](alpha) over a wide range of surfactant concentrations. The value of a was a function of the block copolymer composition irrespective of the individual block lengths: it was 1 for block copolymers with a poly(acrylic acid) content lower than 75 mol % and decreased to 0.4 when the hydrophilic content was increased. This trend was correlated with the exchange dynamics of the stabilizer. The results obtained with Various initiator concentrations, temperatures, and ionic strengths corroborated the previous observation that the important point to explain the evolution of ct with the copolymer composition was the competition between nucleation of the micelles and exchange of the block copolymers between the micelles and the continuously created polymer /water interfaces in the system. The time scale of this exchange (which is very fast for small-molecule surfactants) was on the same order of magnitude as the nucleation step for emulsion polymerizations carried out in the presence of block copolymers.