The phase behavior of thin-film blends of polystyrene (PS) and the random copolymer poly(styrene-co-4-bromostyrene) (PBS) was studied with atomic force microscopy (AFM) and small-angle X-ray scattering SAXS). Phase behavior was studied as a function of the PBS and PS degree of polymerization (N), degree of miscibility [controlled via the volume fraction of bromine in the copolymer (f)], and annealing conditions. The Flory-Huggins interaction parameter chi was measured directly from SAXS as a function of temperature and scaled with f as chi = f(2)chi(S-BrS) [where chi(S-BrS) represents the segmental interaction between PS and the homopolymer poly(4-bromostyrene)] Simulations based on the Flory-Huggins theory and chi measured from SAXS were used to predict phase diagrams for all the systems studied. The PBS/PS system exhibited upper critical solution temperature behavior. The AFM studies showed that increasing f in PBS led to progressively different morphologies, from flat topography (i.e., one phase) to interconnected structures or islands. In the two-phase region, the morphology was a strong function of N (due to changes in mobility). A comparison of the estimated PBS volume fractions from the AFM images with the PBS bulk volume fraction in the blend suggested the encapsulation of PBS in PS, supporting the work of previous researchers. Excellent agreement between the phase diagram predictions (based on chi measured by SAXS) and the AFM images was observed. These studies were also consistent with interdiffusion measurements of PBS/PS interfaces (with Rutherford backscattering spectroscopy), which indicated that the interdiffusion coefficient decreased with increasing chi in the one-phase region and dropped to zero deep inside the two-phase region.