The surface compositions of poly(methylphenylsiloxane) (PMPS) homopolymers and poly(methylphenylsiloxane) - polystyrene (PMPS-PS) diblock copolymers have been analyzed using time-of-flight secondary ion mass spectrometry (TOFSIMS). The high surface sensitivity of this method, which probes samples to depths of a few Angstroms, showed the block copolymer samples to have both poly(methylphenylsiloxane) and polystyrene segments present at the surface. These results may be contrasted with the behavior seen for poly(dimethylsiloxane) (PDMS) blocks and blends, where the low surface energy of the -[O-Si(CH3)(2)]- repeat units typically leads to surface segregation and complete PDMS surface coverage is seen. Investigation of the thermal characteristics of the block copolymers by differential scanning calorimetry revealed a single glass transition temperature, indicating a single phase morphology. The surfaces of the block copolymers also showed an enrichment of -Si(CH3)(3) groups, which are the terminal groups of the PMPS, presumably due to the low surface energy of the trimethylsilyl groups.