Kerogens were purified from 26 samples of the Kimmeridge Clay Formation over the full range of maturities pertinent to petroleum generation. Most samples comprised >90% amorphous, p organic matter (AOM). Prior to and during the early phase of petroleum generation, kerogen densities range between 1.18 and 1.25 g cm(-3). During peak and late stage petroleum generation, densities increase to similar to 1.35 g cm(-3) as hydrogen indices decrease from similar to 350 to 50 mg HC/g C. The data are qualitatively consistent with the loss of alkyl carbon from kerogen to petroleum and the increased aromatization of remaining carbon. The density increase observed for AOM contrasts with the data for vitrinite, which exhibits a decrease in density at maturity levels relevant to petroleum generation. The contrasting behavior of AOM and vitrinite is thought to reflect the differing structural composition of the two kerogen types, most obviously the greater initial aromaticity of vitrinite.