The lateral growth rate of CO2 hydrate films at the boundary between CO2-saturated water and liquid CO2 at pressures between 15 and 25MPa were measured using an optical microscope. Growth rates ranged from about 6-15 mm/s, depending on the supercooling, and became smaller at higher pressure. We assumed the rate-determining process to be mainly heat transport from the growing interface. This assumption allowed its to interpret the difference of the growth rates between the higher and lower pressure experiments as being due to a difference in hydrate film thickness. The addition of NaCl to the solution, on the other hand, reduced the lateral growth rate. In addition, at a NaCl concentration of 10.1 wt%, the lateral growth rates were remarkably lower than those in lower concentration solutions. These observations suggest that the hydrate growth rate in the NaCl solution is determined by not only the heat transportation, but also the mass transportation of NaCl that might arise from a build up of NaCl at the growing interface. (C) 2002 Elsevier Science B.V. All rights reserved.