Mass transfer from CO2 drops traveling in high-pressure and low-temperature water, simulating the process of CO2 ocean disposal, was investigated in a water tunnel. This investigation covered cases for CO2 drops with and without a hydrate shell. In both cases, the CO2 drops dissolved in water, indicating that the crystal hydrate shell did not stop mass transfer. Influence of the water flow on mass transfer was studied. The drop-shrinkage rate was noticed to increase with increase in the water velocity; and, the rate of mass transfer from CO2 drops with a hydrate shell was found to be much smaller than that from CO2 drops without a hydrate shell under the same flow condition. Coefficients for mass transfer from CO2 drops with and without a hydrate shell were analyzed. In the former case, the mass-transfer coefficient was evaluated on the basis of a series-mass-transfer model; in the latter case, a conventional-type correlation for the mass-transfer coefficient was developed.