Sequential infiltration synthesis (SIS) represents a simple and straightforward approach to grow inorganic materials in polymeric films. In this work a combination of in situ and ex situ spectroscopic ellipsometry (SE) analysis is used to provide a comprehensive picture of polymer film evolution and Al2O3 incorporation as a function of the number of SIS cycles. Two different growth regimes can be clearly distinguished. Initially, Al2O3 incorporation determines a marked swelling of polymer films during each SIS cycle. Subsequently, no significant variation of the polymer film thickness is observed by means of in situ SE, despite Al2O3 mass uptake at each SIS cycle being clearly highlighted by the gradual increase of the refractive index observed by in situ SE as well as by the regular increment of Al2O3 film thickness detected by ex situ SE. The experimental data suggest that after a few SIS cycles, Al2O3 growth in the …