Miscibility phase behavior in blends of poly(bromostyrene) with polystyrene (PS) has been investigated by means of time-resolved light scattering, optical microscopy, and DSC. Cloud point phase diagrams of blends of conventional PS with poly-(2-bromostyrene) (P2BrS), poly-(3-bromostyrene), and poly-(4-bromostyrene) of comparable molecular weights were established by light scattering. Of particular interest is the fact that ortho, meta, and para substitutions in the styrenic aromatic rings of poly(bromostyrene) show profound effects on the composition-temperature phase diagrams of their blends with PS, exhibiting a lower critical-solution temperature (LCST), an upper critical solution temperature (UCST), and combined LCST/UCST diagrams, respectively. Poly-(2-chlorostyrene) exhibits an LCST behavior very close to that of the P2BrS blend, suggesting that these types of halogen atoms may be inconsequential to phase behavior. A similar study has been extended to a PS blend containing commercial brominated PS (66 mol % bromine substitution) to determine what location of bromine substitution is crucial for miscibility enhancement in the flame-retardant brominated PS blends.