[en] Our direct knowledge of the rates of gas exchange in lakes and the ocean is based almost entirely on measurements of the isotopes ¹⁴C, ²²²Rn and ³He. The distribution of natural radiocarbon has yielded the average rate of CO₂ exchange for the ocean and for several closed basin lakes. That of bomb produced radiocarbon has been used in the same systems. The ²²²Ra to ²²⁶Ra ratio in open ocean surface water has been used to give local short term gas exchange rates. The radon method generally cannot be used in lakes, rivers, estuaries or shelf areas because of the input of radon from sediments. A few attempts have been made to use the excess ³He produced by decay of bomb produced tritium in lakes to give gas transfer rates. The uncertainty in the molecular diffusivity of helium and in the diffusivity dependence of the rate of gas transfer holds back the application of this method. A few attempts have been made to enrich the surface water of small lakes with ²²⁶Ra and ³H in order to allow the use of the ²²²Rn and ³He methods. While these studies give broadly concordant results, many questions remain unanswered. The wind velocity dependence of gas exchange rate on molecular diffusivity also remains in limbo. Finally, the degree of enhancement of CO₂ exchange through chemical reactions has been only partially explored. These remaining uncertainties have relevance to some of the key applications of our knowledge regarding gas exchange rates to environmental problems