Five polystyrene-poly(dimethylsiloxane) (PS/PDMS) diblocks were synthesized by sequential anionic polymerization, and their morphologies characterized by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). All materials are microphase-separate in toluene-cast films, and estimates of the interaction parameter chi indicate that these materials are all strongly segregated. The experimentally determined phase diagram is strongly skewed towards low styrene volume fractions, even more than the styrene-isoprene (SI) diblock phase diagram, even though little conformational asymmetry should exist in the PS/PDMS system. The PS/PDMS diblocks form substantially larger microdomain structures than the analogous SI diblocks, reflecting the stronger segregation strength.