An optical measurement technique, which is based on the Foerster resonant energy transfer (FRET) between two different dye molecules, has been applied successfully to observe volume expansion of a liquid solution, when it is pressurized with CO2. Rhodamine-B and Rhodamine-700 were dissolved in ethanol to form the FRET active dye solution. In a first "prove of principle" experiment, the sensitivity of the FRET efficiency towards volume expansion was demonstrated by pressurizing the liquid dye solution in a cuvette with CO2. From the rise of the meniscus of the solution inside the cuvette as a function of CO2 pressure, the simultaneously acquired FRET spectra could be correlated with the volume expansion of the dye solution. In a second experiment, the dye solution was injected into CO2 at different supercritical antisolvent operation pressures. FRET spectra were recorded 3 mm downstream of the injector nozzle, always upstream of the breakup of the injected liquid solution. At pressures below the thermodynamic mixture critical pressure (7.9 MPa @ 313 K) of the system ethanol/CO2 no liquid phase volume expansion was observed. At pressures between the thermodynamic and the dynamic mixture critical pressure (8.5 MPa @ 313 K) of the same system, volume expansion could be evidenced before the breakup of the injected liquid solution. (C) 2011 Elsevier B.V. All rights reserved.