Models describing the evolution of the partial pressure of atmospheric oxygen over Phanerozoic time are constrained by the mass balances required between the inputs and outputs of carbon and sulfur to the oceans. This constraint has Limited the applicability of proposed negative feedback mechanisms for maintaining Levels of atmospheric O-2 at biologically permissable Levels. Here we describe a modeling approach that incorporates O-2-dependent carbon and sulfur isotope fractionation using data obtained from Laboratory experiments on carbon-13 discrimination by vascular Land plants and marine plankton. The model allows us to calculate a Phanerozoic O-2 history that agrees with independent models and with biological and physical constraints and supports the hypothesis of a high atmospheric O-2 content during the Carboniferous (300 million years ago), a time when insect gigantism was widespread.