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 miscibility with MSSQ. As the P(MMA-co-DMAEMA) loading level increases, the excimer-to-monomer (I-E/I-M) ratio also increases, suggesting that the MSSQ/P(MMA-co-DMAEMA) hybrid nanocomposites move toward greater immiscibility. This ratio approaches that of the neat polymer for domain sizes > 5 nm as determined by small-angle neutron scattering (SANS). The fluorescence results also show that the lower the initial silanol content in MSSQ, the greater the immiscibility of the MSSQ and porogen, which ultimately translates into larger pores upon porogen burnout, as evidenced by the transmission electron microscopy results.