Modulated differential scanning calorimetry (MDSC) and fluorescence spectroscopy have been used to study the miscibility of methylsilsesquioxane (MSSQ)/poly(methyl methacrylate-co-(dimethylamino)ethyl methacrylate) [P(MMA-co-DMAEMA)] hybrid nanocomposites, which are promising candidates for spin-on, ultralow dielectric constant materials. MSSQ resins with different initial concentrations of −SiOH (silanol) are used to study the effect of end-group functionality on the phase separation behavior of the hybrid nanocomposites. MDSC measurements, which are useful for studying phenomena with length scales of 10 nm and larger, are consistent with apparent miscibility for P(MMA-co-DMAEMA) loading levels as high as 70 wt % for both resins studied. In contrast, fluorescence studies on pyrene-labeled PMMA derivatives, which interrogate substantially smaller distance scales, reveal that pyrene excimer formation occurs at P(MMA-co-DMAEMA) loading levels above 6 wt %, thus establishing an upper limit on local miscibili...