Because of the relative constancy of pH in most subaqueous sediments and the general lack of measurability of Eh, these parameters are not practically useful for classifying sedimentary environments. In their place a new classification is offered which, derived from studies of modern sediments, is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation. Sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen. Anoxic environments, in turn, are divided into sulfidic and non-sulfidic depending upon the presence of measurable dissolved sulfide. Anoxic-nonsulfidic environments are further divided into postoxic, or resulting from oxygen removal without sulfate reduction (weakly reducing), and methanic , or resulting from complete sulfate reduction with consequent methane formation (strongly reducing). The environments are shown to succeed one another during early diagenesis in the order: oxic, post-oxic, sulfidic, methanic. Iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record.