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Reconstructing the history of biological productivity and atmospheric oxygen partial pressure ( p O 2 ) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating p O 2 and productivity during the Proterozoic. O-MIF, reported as Δ′ 17 O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS 2 ) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air–sea gas exchange. Previous analyses of these data concluded that p O 2 at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on p O 2 is essentially unconstrained by these data. Indeed, p O 2 levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that p CO 2 could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O 2 ) between the atmosphere and surface ocean is controlled more by air–sea gas transfer rates than by biological productivity. Improved estimates of p CO 2 and/or improved proxies for Δ′ 17 O of atmospheric O 2 would allow tighter constraints to be placed on mid-Proterozoic p O 2 .