It has been widely considered that the global warming, induced by the increasing concentration of carbon dioxide and other greenhouse gases in the atmosphere, is an environmental task affecting the world economic development. In order to mitigate the concentration Of CO2 in the atmosphere, the sequestration of carbon dioxide into the ocean had been investigated theoretically and experimentally over the last 10 years. In addition to ocean dynamics, ocean geological, and biological information on large space and long time scales, the physical-chemistry properties of seawater-carbon dioxide system at high pressure (P > 5.0 MPa) and lower temperature (274.15 K < T < 280.15 K) (the deep ocean environment) is very important for both fundamental and engineering investigations. The density of carbon dioxide seawater solution is one of the key physical properties, which is an indispensable property for numerically estimating the dynamic evolution of carbon dioxide enriched seawater plume. In this study, the density of carbon dioxide seawater solution was measured by applying Mach-Zehnder Interferometry to a visible high-pressure vessel experimental system. The preliminary experimental results reveal that (1) the density of carbon dioxide seawater solution increases with the increase of mass fraction of carbon dioxide in its seawater solution at a pressure ranging from 4.0 to 12.0 MPa and temperatures of 276.15 and 283.15 K; (2) the density deference between carbon dioxide seawater solution and pure seawater is linearly proportional to the mass fraction of carbon dioxide and independent on pressure, temperature, and salinity; (3) the slope of this linear function of density deference with respect to carbon dioxide mass fraction is 0.273 g/cm(3), which is approximately same with that of carbon dioxide freshwater solution, the slope of which is 0.275 g/cm(3) (c) 2004 Elsevier Ltd. All rights reserved.