During oil shale pyrolysis at high temperature, the conversion of kerogen to oil and gas proceeds dominantly through a mechanism involving bitumen as a reaction intermediate-at low maturities bitumen is formed from decomposition of kerogen, while at high maturities bitumen is transformed primarily to oil and gas. Here, we study the chemical composition of Green River bitumens of a range of maturities by high-field C-13 nuclear magnetic resonance (NMR) spectroscopy. Numerous trends in the evolution of bitumen with maturity are observed, some of which are similar to trends previously observed in kerogen while others are not. As found previously for kerogen, bitumen becomes more aromatic (less aliphatic) with increasing maturity. However, in contrast to kerogen, which is primarily consumed during maturation and was found previously to have aliphatic chains that become shorter and/or more branched with maturity, aliphatic chains in bitumen lengthen with maturity in the low maturity regime where bitumen is primarily being formed but then shorten with maturity in the high maturity regime where bitumen is primarily being consumed. The structure of aromatic rings in bitumen is essentially unchanged with maturity, as their size, alkyl substitution, and heteroatom substitution are found to be independent of maturity; in contrast, the size of aromatic rings in kerogen generally increases With maturity. These measurements of the chemical composition of the bitumen intermediate enhance understanding of petroleum generation by oil shale pyrolysis at high temperature.