Amphiphilic block copolymers consisting of a hydrophobic block of hydrogenated polybutadiene and a partly sulfonated polyelectrolyte block of poly(styrenesulfonate) are effective stabilizers in emulsion polymerization. At low relative amounts of block copolymer (0.4 wt % as a fraction of monomer weight) and salt-free conditions, well-defined and stable latices with particle diameters of ca. 100 nm and solid contents of 20 wt % are obtained. This high stabilization efficiency of optimized block copolymer systems enables the formulation of latex systems with a relatively low remaining polarity in solid films and offers new interesting model systems with exclusively electrosteric stabilization. A comparison of polymerization in high or low ionic strength solution and the variation of the degree of sulfonation of the poly(styrenesulfonate) block shows an optimum stabilization of the latices at low ionic strength during polymerization. Fully sulfonated polymers systems presumably show a molecular orientation perpendicular to the particle surface, whereas 50% sulfonated species take on a traillike conformation along the surface, which is explained by remaining hydrophobic interactions. Due to this multiple surface particle contacts, the application of partly sulfonated polymers leads to more effective stabilization. At higher stabilizer concentrations, aggregates are found, which can be redispersed by ultrasonification or addition of low molecular weight surfactant solution.