The phase behavior in ternary blends of polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(styrene-co-methyl methacrylate) has been determined as a function of sequence distribution at 150 degrees C. Two copolymers were investigated which have comparable compositions (48-50 mol % methyl methacrylate monomer units) but distinct sequence distributions, namely strictly alternating and random. The phase behavior was determined via transmission electron microscopy, which found the number of phases, the relative amounts of the phases, and the spatial arrangement of the phases. Equilibrium blend compositions were calculated from the experimental results using mass balance equations. These equilibrium compositions were used to construct isothermal ternary phase diagrams which exhibited three pairwise immiscible binary systems and a three-phase region. From these ternary phase diagrams, we show that the compositionally symmetric copolymers are more miscible in PMMA than PS and that this asymmetry in miscibility is more pronounced with the alternating sequence. This sequence distribution effect is discussed in terms of specific interactions between the methyl methacrylate monomer units. With either alternating or random copolymers, the three-phase blends exhibit encapsulation when the copolymer-rich phase is less than 25 vol % of the blend; otherwise the copolymer-rich phase forms a continuous matrix. The equilibrium compositions of binary blends containing poly(methyl methacrylate) and a copolymer were also investigated as a function of molecular weight.