Differential thermal and dynamic mechanical analysis (DTA and DMA) were carried out on the blend system of atactic poly(methyl methacrylate) (PMMA) with poly(p-t-butyl phenol formaldehyde) (PTBF). In both the techniques, the PMMA/PTBF blend system exhibited a composition-dependent, single glass-transition temperature, indicating miscibility. The blends exhibited composition-dependent morphology dominated by induced cluster formation of PMMA as a separate phase in blends. The DTA scans of blends exhibited broad melting endotherms. Also, in DMA, apart from the main relaxation (tan delta), the system showed an additional high-temperature relaxation (T-u), indicating the presence of a separate phase. In addition, wide-angle X-ray diffraction (WAXD), Fourier-transform infrared (FTIR) spectroscopy, and sonic pulse propagation measurements provide evidence for the formation of PMMA clusters in the bulk PMMA/PTBF blends. The mechanical behavior of the blends is discussed in terms of the variations in shape and magnitude of tan delta, E’, and E" curves. It is concluded that PTBF acts as a compatible plasticizer in PMMA/PTBF blends and, because of changed morphology, improves the elongation of blend films compared to pure PMMA.