A proposed mechanical model based on our modification of the conventional Takayanagi model was successfully used to describe the dynamic mechanical modulus data of binary blends of a high-temperature thermosetting polycyanate (Bisphenol E dicyanate) with thermoplastics (polysulfone or poly-(ether imide)). The model parameters were found to quantitatively agree well with the physical compositions of the phases in the blends. The modeling results suggested interesting shifts in the phase continuity of the blends as a function of the thermoplastic contents. At low thermoplastic contents (10% and below), the blends exhibited a simple continuous-dispersed phase morphology, with the continuous polycyanate phase surrounding the dispersed phase. As the thermoplastic contents increased (20 vol % and above), a notable change in the phase composition and morphology was observed in the dispersed domains, and a continuous thermoplastic phase started to form, while the polycyanate continuous phase retreated rapidly. The findings from the modeling were also further experimentally substantiated by optical and electron microscopy observations.