The anionic dispersion copolymerization of styrene and butadiene was investigated. The suitability of the lower alkanes butane and pentane as dispersing media was tested. Only pentane turned out to be suited for the synthesis of non-aggregated polymer particles. Following block copolymerization experiments, the active chain ends were found to be homogeneously distributed over the whole particle volume by TEM analysis of OsO4 treated polymer probes. This result contradicts the opinion of Kim et al. [Korea Polym J7 (1999) 64] who suppose a location of the active centers only at the surface of the polymer particles. Polymer particle aggregation in the synthesis of poly(styrene-b-butadiene) and poly(styrene-b-butadiene-b-styrene) di- and triblock copolymers can be attributed during the early part of butadiene polymerization to the well known association of growing lithium chain ends from separated polymer. Later on, at higher butadiene conversions, an entanglement of longer polymer chains causes agglomeration, too. The most important parameter to obtain a stable poly(styrene-b-butadiene)lithium dispersion is the molecular mass of the poly(butadiene) block. The upper limit was evaluated to be 8000 g/mol. For technical applications, this means a maximum of 5 wt% of poly(butadiene) for a total molecular mass of 150,000 g/mol.