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In 1983, Thiemens and Heidenreich (Science 219, 1073) reported the discovery of a mass-independent isotope process in oxygen. Subsequently, mass-independent isotopic compositions were observed in meteorites, atmospheric O-2, CO2, O-3, H2O2, CO, N2O, and aerosol sulfate and nitrate. In addition, solid samples from the Namibian Desert, Antarctic dry valleys, Miocene volcanic ash, and Death Valley varnishes all possess mass-independent isotopic compositions that have permitted new scientific insights to be obtained. Sulfur mass-independent isotopic anomalies in Precambrian samples have provided a completely new mechanism to track atmospheric O-2 levels and the evolution of life from similar to3.9 to 2.2 x 10(9) years ago. Measurements of mass-independent isotopic compositions in Martian meteoritic sulfates and carbonates have also provided new information on Martian atmosphere-regolith interactions and history.

In sum, the use of mass-independent isotopic compositions has advanced a wide range of geo- and cosmo-chemical frontiers. In addition, these newly discovered effects have provided a new means to probe the chemical physics of molecular transition state theory.