Fast measurements of the interfacial tension (IFT) of the water + carbon dioxide system at 298 K and 6.6 MPa were conducted by a modified pendant drop method to investigate the influence of nonequilibrium composition during the mass transfer of CO2 into the water drops. An apparent decrease in interfacial tension could be attributed to the use of equilibrium densities in their determination. Using the density gradient as it occurs during mass transfer by the measurable volume increase due to CO2 uptake, we found that the interfacial tension is constant all of the time. For this reason, it is vital to take the density gradient into account if the time-dependent interfacial tension is measured. This gradient can also be used to determine densities in nonequilibrium compositions. The uncertainty of the density determination for the unsaturated water density in the water + carbon dioxide system is strikingly low, lower than 1%.