Sulfur hexafluoride has been widely used in field studies and laboratory experiments to develop a relationship between gas transfer and wind speed. The interpretation of the data from such studies requires the diffusion coefficient of SF6 (DSF6), which has not previously been measured. In this study, DSF6 has been determined in pure water and in 35% NaCl over a temperature range of 5--25¿C. The measurements were made using a continuous-flow diffusion cell where SF6 flows beneath an agar gel membrane while helium flows above the gel. The experimental data for pure water yielded the following equation: DSF6=0.029 exp (-19.3/RT, where R is the gas constant and T is temperature in kelvins). Measurements of DSF6 in 35% NaCl were not significantly different for the pure water values. On the basis of our data, we estimate the Schmidt numbers for seawater over the temperature range 5--25¿ C to be Sc=3016.1-172.00t4.4996t2- 0.0047965t3, where t is temperature in degrees Celsius. ¿American Geophysical Union 1995