On the basis of the sulfur isotope age curve of marine sulfate evaporites and currently available numerical data for the sulfur cycle the stationary terrestrial sulfate reservoir has been calculated as a function of time for the last 600¿106 yr, this also giving information on the partial inventory of photosynthetic oxygen locked up in sedimentary and marine SO4 2. The results obtained confirm previous concepts (arrived at by geochemical flux models) of a sizable interchange of material between both sedimentary sulfur reservoirs and, accordingly, varying amounts of oxygen stored as sedimentary sulfate (this amount fluctuating between some 0.5 and 1.5¿1022 g during the Phanerozoic). With the constancy of the sedimentary Δ13Cearb record indicating that the crustal Corg resorvoir and its stoichiometric oxygen equivalent (i.e., total photosynthetic oxygen) did not change very much during the last quarter of the earth's history, fluctuations in sulfate oxygen must have been buffered by the two other O2 inventories, namely oxygen fixed as Fe(III) oxide and free oxygen of the atmosphere. Although the main responsibility for balancing variations in sedimentary sulfate would go to the fixed oxygen because of its overwhelming size, repercussions on the free oxygen can by no means be excluded. If we accept a variation of free oxygen proportionate to that of the sum of the two reservoirs as a minimum requirement, then atmospheric P02 can be expected to have fluctuated during the Phanerozoic within some ¿20% of its present level.