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Nanofluids have novel characteristics that make them potentially useful for different applications. Realizing modest mass transfer enhancement in conventional nanofluids, in this study, mass-transfer of carbon dioxide in pure water and water-based nanofluids dispersed with silica nanoparticles at different initial pressures up to 15 MPa and at temperatures of 35 °C and 45 °C was investigated. Deionized water and two nanofluids at different concentrations with volume of 150 cm3 were used for this purpose. CO2 was brought in contact with each solution in a pressure-volume-temperature (PVT) cell with no mixer. Additionally, carbon dioxide diffusion coefficients at different pressures were estimated based on Fick's law. The obtained results demonstrated that water/silica nanofluid with 0.5 wt% and 0.1 wt% increased the carbon dioxide diffusion coefficient up to 39.2% and 11.9% compared to that in pure distilled water, respectively. Moreover, it was observed that the measured diffusion coefficient of carbon dioxide in water increased with temperature rise from 35 °C to 45 °C at constant pressure. However, it could be seen that, the diffusion coefficient decreased with pressure at constant temperature. It was concluded that among the enhancement mechanisms of nanoparticles, (i.e. grazing effect and Brownian motion), Brownian motion would play the main role in mass transfer enhancement. Keywords: Nanofluid, Mass transfer enhancement, Carbon dioxide, Diffusion coefficient, Brownian motion