Through the Deep Sea Drilling Project samples of interstitial solutions of deeply buried marine sediments throughout the World Ocean have been obtained and analyzed. The studies have shown that in all but the most slowly deposited sediments pore fluids exhibit changes in composition upon burial. These changes can be grouped into a few consistent patterns that facilitate identification of the diagenetic reactions occurring in the sediments. Pelagic clays and slowly deposited (<1 cm/10³yr) biogenic sediments are the only types that exhibit little evidence of reaction in the pore waters. In most biogenic sediments sea water undergoes considerable alteration. In sediments deposited at rates up to a few cm/10³ yr the changes chiefly involve gains of Ca²⁺ and Sr²⁺ and losses of Mg²⁺ which balance the Ca²⁺ enrichment. The Ca-Mg substitution may often reach 30 mM/kg while Sr²⁺ may be enriched 15-fold over sea water. These changes reflect recrystallization of biogenic calcite and the substitution of Mg²⁺ for Ca²⁺ during this reaction. The Ca-Mg-carbonate formed is most likely a dolomitic phase. A related but more complex pattern is found in carbonate sediments deposited at somewhat greater rates. Ca²⁺ and Sr²⁺ enrichment is again characteristic, but Mg²⁺ losses exceed Ca²⁺ gains with the excess being balanced by SO₄^{post staggered2-} losses. The data indicate that the reactions are similar to those noted above, except that the Ca²⁺ released is not kept in solution but is precipitated by the HCO₃^{post staggered-} produced in SO₄^{post staggered2-} reduction. In both these types of pore waters Na⁺ is usually conservative, but K⁺ depletions are frequent. In several partly consolidated sediment sections approaching igneous basement contact, very marked interstitial calcium enrichment has been found (to 5.5 g/kg). These phenomena are marked by pronounced depletion in Na⁺, Si and CO₂, and slight enhancement in Cl^-. The changes are attributed to exchange of Na⁺ for Ca²⁺ in silicate minerals forming from submarine weathering of igneous rocks such as basalts. Water is also consumed in these reactions, accounting for minor increases in total interstitial salinity. Terrigenous, organic-rich sediments deposited rapidly along continental margins also exhibit significant evidences of alteration. Microbial reactions involving organic matter lead to complete removal of SO₄^{post staggered2-}, strong HCO₃^{post staggered-} enrichment, formation of NH₄^{post staggered+}, and methane synthesis from H₂ and CO₂ once SO₄^{post staggered2-} is eliminated. K⁺ and often Na⁺ (slightly) are depleted in the interstitial waters. Ca²⁺ depletion may occur owing to precipitation of CaCO₃. In most cases interstitial Cl^- remains relatively constant, but increases are noted over evaporitic strata, and decreases in interstitial Cl^- are observed in some sediments adjacent to continents.