Oxidative weathering of organic carbon in sedimentary rocks is a major source of CO2 to the atmosphere over geological timescales, but the size of this emission pathway in Earth's past has not been directly quantified due to a lack of available proxy approaches. We have measured the rhenium isotope composition of organic-rich rocks sampled from unweathered drill cores and weathered outcrops in south Texas, whose stratigraphic successions can be tightly correlated. Oxidative weathering of more than 90% of the organic carbon and ∼85% of the rhenium is accompanied by a shift to lower rhenium isotope compositions in the weathered outcrops. The calculated isotope composition of rhenium weathered from the initial bedrock for individual samples varies systematically by ∼0.7‰ with different fractions of rhenium loss. This variation can be empirically modeled with isotope fractionation factors of α = 1.0002–1.0008. Our results indicate that the isotope composition of rhenium delivered to the oceans can be altered by weathering intensity of rock organic matter and that the rhenium isotope composition of seawater is sensitive to past oxidative weathering and associated CO2 emissions.