The interaction between a polystyrene- b -polybutadiene- b -polystyrene (SBS) porogen and a low-k methylsilsesquioxane (MSQ) matrix under different curing profiles and their impact on porogen size were studied by grazing incidence small-angle X-ray scattering, viscosity measurement, and Fourier transform infrared analysis. For slow curing, significant porogen diffusion and aggregation occurred between 100 and 170°C, at which the porogen size increased from 12 to 32 nm. The interaction mechanism between porogen and the MSQ matrix and its correlation with porogen size during the thermal cure process were elucidated. The aggregation of the SBS porogen was greatly influenced by the microstructure of the MSQ matrix at three controlling temperatures; namely, (1) glass transition temperature, Tg (∼100°C), (2) onset temperature (160°C) for transforming cage to network structure, and (3) immobilization temperature (170°C). In contrast, a rapid curing rate enabled the formation of small porogen size (∼12 nm) in the hybrid films and yielded better mechanical strength.