This study used refractometry, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, and dielectric analysis to assess the viscoelastic properties and phase behavior of blends containing 0-20% (w/w) 12-tert-butyl ester dendrimer in poly(methyl methacrylate) (PMMA). Dendritic blends were miscible up through 12%, exhibiting an intermediate glass-transition temperature (T-g; alpha) between those of the two pure components. Interactions of PMMA C=O groups and dendrimer N-H groups contributed to miscibility. T-g decreased with increasing dendrimer content before phase separation. The dendrimer exhibited phase separation at 15%, as revealed by Rayleigh scattering in ultraviolet-visible spectra and the emergence of a second T-g in dielectric studies. Before phase separation, clear, secondary p relaxations for PMMA were observed at low frequencies via dielectric analysis. Apparent activation energies were obtained through Arrhenius characterization. A merged ap process for PMMA occurred at higher frequencies and temperatures in the blends. Dielectric data for the phase-separated dendrimer relaxation (cu,) in the 20% blend conformed to Williams-Landel-Ferry behavior, which allowed the calculation of the apparent activation energy. The a, relaxation data, analyzed both before and after treatment with the electric modulus, compared well with neat dendrimer data, which confirmed that this relaxation was due to an isolated dendrimer phase.