The PVTx behaviour for the carbon dioxide (CO2) + pentafluoroethane (R125) system was measured by the Burnett method within the range 323-363 K and at pressures from 10 to 5000 kPa. The apparatus was calibrated by means of helium and its performance was checked by measurements of PVT for the mixture components. Their second and third virial coefficients were found to be in good agreement with data available in the literature. A new experimental procedure for assessment of the mixture composition has been implemented, based on gas chromatographic analyses of the sample collected during expansions. The derived second and third virial coefficients and respective cross virial coefficients were compared also with correlating methods adapted from the literature. No experimental data on the PVTx for the CO2 + R 125 system could be found in the literature.