The first in-situ two-dimensional grazing incidence small-angle X-ray scattering (2D GISAXS) study on the evolution of nanopores during the thin film formation of porous dielectrics from composite films is reported. A soluble poly(methylsilsesquioxane) (PMSSQ) precursor and a four-armed poly(e-caprolactone) (PCL4) were chosen as the model matrix and porogen components within the composite film. The measured 2D GISAXS data were analyzed quantitatively using a GISAXS formula derived under the distorted wave Born approximation. It is shown that in-situ GISAXS is a powerful tool for monitoring the evolution of nanopores in dielectric thin films, providing structural characteristics such as size, size distribution, shape, electron density, and porosity, all as a function of temperature and time. In addition, the mechanism for forming imprinted nanopores in the dielectric films by sacrificial thermal degradation of the porogen was determined by in-situ GISAXS analysis. Phase separation of the PCL4 porogen was i...