In this work UV-vis spectroscopy is used to measure in situ the volume expansion of the saturated liquid phase during gas-antisolvent (GAS) processes with compressed CO2. The technique involves the use of two internal standards: potassium hexacyanoferrate(III) is used for polar solvents, and ferrocene for nonpolar solvents. The absorbance peaks of the reference substances are used to determine the volume expansion of the saturated liquid phase. Measurements on the dimethyl sulfoxide + CO2, dioxane + CO2, and dimethyl sulfoxide + water + CO2 systems have been made in the temperature range from 298 to 313 K and for pressures up to 70 bar. The experimental results obtained compare well with available literature data.