Recent SAXS measurements have indicated that CO2 can dramatically swell the surfactant layer in w/o microemulsions formed in CO2-swollen oils and also enhance solubilisation levels. This is a surprising result since CO2 has very poor compatibility with most hydrocarbon surfactants. Our proposal concerns systematic SANS studies to quantify the extent of interfacial expansion through droplet size measurements. The extent of swelling and location of CO2 in the interfacial layer will also be examined through contrast profile analysis both for microemulsion systems and also nanoparticles for which the total interfacial area is not free to expand.