The purpose of this study was to provide experimental evidence of the separation of the polymer components at different scales during conventional processing. This was achieved by characterizing the surface and the bulk (cross section) of moldings manufactured with a high-flow grade and a low-flow grade of commercial polystyrene by the time-of-flight secondary ion mass spectrometry (TOF-SIMS) analytical technique. Owing to the geometric constraints of the mold used, a weld was also obtained. Different surface spectra were observed for the two molded polystyrenes. The surface of the high-flow grade moldings showed the spectral features of low-molar polyolefin (paraffin) contaminants, whereas the bulk was dominated by polystyrene. Spectra from both the surface and the bulk of the low-flow grade moldings were characteristic of polystyrene. Mold-filling effects on the surface composition were observed in the flow front region of molded short-shots of the low-flow grade. The spectral changes indicated the abundance, in the surface, of the high end of the molar-mass distribution of the material during the mold tilling process. Two-dimensional maps of the secondary ions from the low-flow grade also showed an occasional alkali contamination, preferentially along the notch of the weld.