The local swelling through the thickness of a polymer film induced by exposure to carbon dioxide is assessed using in situ selective modification of a swollen amphiphilic polymer template. The swollen film is selectively modified by condensation of tetraethylorthosilicate within the hydrophilic domains. Monitoring the size of pores resulting from the unmodified hydrophobic domains provides a measure of the local CO2 concentration within the film. A maximum in the average pore size is obtained near the density fluctuation ridge with a gradient in pore size extending approximately 150 nm into the film from the free surface and a short (< 10 nm) gradient at the buried interface. Far from this maximum, the pore size through the thickness of the film is uniform.