A new method is presented for recognizing the degree of bottom-water oxygenation in organic carbon-bearing rock. The method uses the parameter Degree of Pyritization (DOP), which is the ratio of pyritic Fe to pyritic Fe + acid-soluble Fe. Sediments with ages ranging from Devonian to Cretaceous have been classified on paleoecological and sedimentological criteria as having been deposited in aerobic conditions (deposited in fully oxygenated bottom waters); restricted marine conditions (deposited in waters with low oxygen concentrations); or inhospitable bottom conditions (where little or no oxygen is present, and H 2 S may be continually or intermittently present). Aerobic (DOP < 0.42) can be clearly separated from restricted samples (0.46 < DOP < 0.80), but the latter have some overlap with the inhospitable bottom category (0.55 < DOP < 0.93). A boundary at DOP 0.75 separates more than 90 percent of this data. DOP values discriminate between paleoenvironments more effectively than organic carbon contents or the ratio of organic carbon to pyritic sulfur.