[en] In the formation of ozone by an electric discharge in molecular oxygen, substitution of ¹⁸O and ¹⁷O for ¹⁶O produces equal kinetic isotope effects. This may be taken as a paradigm of non-mass dependent isotope effects. However, a number of other reactions have been thus labeled, even when deviations from the expected ratio of ∼0.5 for (¹⁷O/¹⁶O)/(¹⁸O/¹⁶O) are a few parts per thousand. Several equilibrium constants are calculated for isotopic exchange reactions involving oxygen and sulfur isotope effects, and it is shown that exact calculations can differ from this approximate ratio by a few percent. The conclusion is that the label “non-mass dependent” should be applied with care. (author)