The diffusion coefficients D of important gas tracers dissolved in water an seawater were measured with a modified Barrer method. The measurements include the gases He, Ne, Kr, Xe, H2 , CH4 , and CO2 dissolved in distilled water in the temperature range from 5 to 35 ¿C, and He and H2 dissolved in seawater in the same temperature range. The maximum systematic error is estimated to be well below 5%. The isotopic fractionation in the diffusion coefficient, &egr;D, was determined to be (-0.87¿0.05)%o for 13 CO2 /12 CO2 and (15¿3)% for 3 He/4 He. ¿American Geophysical Union 1987