Carbon and hydrogen stable isotope measurements on sapropelic and humic kerogen isolated from laboratory-heated sediments reveal several isotopic trends. An initial depletion of ¹³C and D (²H, or deuterium) in kerogen isolated from the heated sapropelic sediment appears to be the result of isotopic heterogeneity, interaction with soluble organic matter, and/or volatilization of a heat-labile fraction of the protokerogen. During the main isotopic trend the sapropelic kerogen becomes enriched in ¹³C and D by about 5 and 50 ppt (parts per thousand), respectively. Theoretical calculations based on a Rayleigh fractionation process suggest that this trend is largely controlled by the generation of isotopically light methane directly from the ker gen. This argument also applies to the main hydrogen isotopic trend for the humic kerogen which becomes enriched in D by about 90 ppt. The main ¹³C trend for the humic kerogen, however, results in a maximum enrichment in ¹³C of only about 1.6 ppt and appears to be largely controlled by factors other than methane. A final depletion trend in ¹³C and D occurs for the sapropelic kerogen at very high temperatures. This may be due to a reduced isotopic fractionation at higher temperatures, or to the indirect generation of methane from kerogen through a liquid intermediate which is subsequently coked to pyrobitumen.