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 induced below 200 degrees C by cross-linking of the PMSSQ precursor matrix during thermal curing. This process generated porogen aggregates, each individually imprinted pore in the film through thermal degradation; the shape, size, and size distribution of the porogen aggregates are directly reflected in the dimensions of the imprinted pores. Moreover, it was found that higher porogen loadings caused larger porogen aggregates with a greater size distribution. The present results thus show that the structural characteristics of nanopores imprinted within PMSSQ dielectric films are governed by the PCL4 porogen aggregates formed through curing of the PMSSQ precursor matrix.