Insoluble organic matter (kerogen) was isolated from a suite of 23 marine shales and carbonate rocks of early Proterozoic to Middle Cambrian age from Australia. The stable carbon isotopic composition, δC13PDB, of the organic matter appears to vary with late diagenetic or incipient metamorphic rank as given by the hydrogen-to-carbon atomic ratio. Geochemical maturation of the kerogen beyond a rank equivalent to 91 to 93 percent carbon results in marked enrichment of the residual organic matter in C13. Hence, low-rank (unmetamorphosed) organic matter is isotopically lighter (δC13PDB range = −31.3 to −18.6 per mil, mean = −26.9 per mil) than high-rank (metamorphosed) material (δC13PDB range = −25.8 to −10.8 per mil, mean = −17.2 per mil). Kerogens from two Precambrian evaporitic sedimentary rocks are isotopically heavier than normal marine organic matter of similar rank. New isotopic and rank data from 11 carbonaceous, chert samples from the Onverwacht Group, Swaziland Sequence, South Africa, indicate that postdepositional thermal alteration may account for the anomalously heavy reduced carbon in the Theespruit Formation.