This work demonstrates the advantage of spin‐contrast‐variation neutron reflectivity (NR) for the structure analysis of a buried interface in a bilayer film comprising organic and inorganic layers of acrylic urethane resin and methylated silica (MePDS), derived from methyl‐group‐substituted perhydropolysilazane. As the proton polarization PH changed from 0 to ±24%, the NR curve of the bilayer film varied significantly. These NR curves were not reproduced using global fitting with a standard bilayer model. The oscillation in the NR curve at PH = −24% was shifted slightly and non‐negligibly from the fitting curve using the best‐fit structure parameters for the curve at PH = 0%. It was found from the shift of the oscillation that the density of the MePDS layer decreased within several nanometres of the interface, but the resin did not permeate the low‐density MePDS layer.