Three hundred new samples of marine evaporite sulfate, of world-wide distribution, were analyzed for δ³⁴S, and 60 of these also for δ¹⁸O in the sulfate ion. Detailed δ³⁴S age curves for Tertiary—Cretaceous, Permian—Pennsylvanian, Devonian, Cambrian and Proterozoic times document large variations in δ³⁴S. A summary curve forδ¹⁸O also shows definite variations, some at different times than δ³⁴S, and always smaller. The measured δ³⁴S and δ¹⁸O correspond to variations in these isotopes in sulfate of the world ocean surface. The variations of δ¹⁸O are controlled by input and output fluxes of sulfur in the ocean, three of which are the same that control δ³⁴S: deposition and erosion of sulfate, and deposition of sulfide. Erosion of sulfide differs in its effect on the S and O systems. δ¹⁸O in the sulfate does not seem to be measurably affected by equilibration with either seawater or with subsurface waters after crystallization. In principle, the simultaneous application of both δ³⁴S and δ¹⁸O age curves should help reduce the number of assumptions in calculations of the cycles of sulfur and oxygen through geological time, and a new model involving symmetrical fluxes is introduced here to take advantage of the oxygen data. However, all previously published models as well as this one lead to anomalies, such as unreasonable calcium or oxygen depletions in the ocean—atmosphere system. In addition, most models are incapable of reproducing the sharp rises of the δ³⁴S curve in the late Proterozoic, the Devonian and the Triassic which would be the result of unreasonably fast net sulfide deposition. This fast depletion could result from an ocean that has not always been mixed (as previously assumed in all model calculations).