Heating of sedimentary rocks under controlled conditions (28°C/minute) causes a sequential, two-stage release of organic compounds. Material released during the 30 to 400°C heating interval corresponds to volatile compounds preexisting as such in the rock. At higher temperatures (400 to 800°C), nonvolatile organic matter thermally decomposes to yield volatile compounds. For a given sample weight, the integrated response of the hydrogen-flame-ionization detector during the 30 to 400°C heating period is directly proportional to the concentration of extractable heavy hydrocarbons in that sample, as independently determined by conventional methods. Similarly, the detector response during the 400 to 800°C heating period is approximately proportional to the organic-carbon content, as determined by a separate combustion analysis. This provides a quantitative evaluation of the organic richness of potential petroleum source rocks. Previous work has shown that the temperature at which the nonvolatile organic matter thermally decomposes depends on the previous temperature history of the rock. Thus thermal analysis provides a means for rapid stimulation of fundamental criteria for source-rock evaluation.