The swelling of thin polymer films by supercritical carbon dioxide has been shown to exhibit an anomalous maximum in the sorption isotherms. This swelling maximum occurs when the activity of the fluid phase approaches unity and the extent of the swelling increases as the critical temperature is approached. Using an in situ selective reactive templating approach based upon silica condensation within hydrophilic domains of an amphiphilic polymer film, the CO2 distribution within the polymer film is elucidated from the pore sizes generated by hydrophobic domains upon removal of the amphiphile. The origin of the maximum in the swelling of the thin films is demonstrated to be concentration gradients that extend from both interfaces of the film; as the temperature approaches the critical temperature, the local swelling at both interfaces increases progressively. However, the pressure range over which gradients in CO2 concentration occurs decreases as the critical temperature for CO2 is approached. (C) 2009 Elsevier B.V. All rights reserved