Abstract The hydrogenous fraction of sediment has been defined by Goldberg (1963) as “composed of those solids which inorganically precipitate from sea water.” As it is often difficult to distinguish between the authigenic fraction of hydrogenous origin and a second authigenic fraction formed during early sediment diagenesis (i.e., diagenesis in the upper few 10s of centimeters of sediment), we have expanded the discussion to include the solidsthat precipitate from pore water of the surface sediment even when such reactions involve the diagenetic alteration of a particulate precursor rather than merely the precipitation of dissolved species. Evidence for these reactions has come from the analysis of pore waters (Gieskes and others, 1975) as well as of the solid products. However, interpretations have often been hampered by inherent problems. For example, pore water of rapidly accumulating shallow-water sediment exhibits strong elemental variations, suggesting dynamic reactions with the enclosing solid phases (Sayles and others, 1973). Unfortunately, the reactive solid products commonly constitute only a minor fraction of the total sediment, their composition and even their presence being masked by a largely nonreactive detrital component (Price, 1976). Within the slowly accumulating sediment of the pelagic environment, the case is reversed. The hydrogenous component can represent a major fraction of the bulk sediment, but reactions are slow enough that elemental gradients within the pore water are diminished by diffusion. Despite these problems, enormous progress has been made in recent years, owing to a vast improvement in analytical techniques.