The Malacca Strait is a shallow passage between the Malay Peninsula and Sumatra with oceanographic and bottom-sediment characteristics closely related to strong currents, debouching rivers, climatic variation, and the close proximity of bordering land masses. The strait assumed its present configuration as a result of the postglacial rise of sea level which drowned the Sunda Shelf. An essentially tidal northwest current flow prevailing in the strait throughout the year is largely responsible for the hydrographic and oceanographic conditions in the region. Surface salinities and temperatures are generally found to be lower than in the surrounding seas. A prominent wedge of cool-temperature, high-salinity bottom water is found to extend from the Andaman Sea into the strait. Bottom sediments primarily consist of muddy sands, with large areas of mud occurring in the vicinity of fora-river mouths and in the Andaman Sea. Calcium carbonate, primarily composed of mollusk shells and miniferal tests, and organic carbon are generally found only in minor amounts in the strait. Higher concentrations of calcium carbonate are confined to local shell deposits and larger percentages of organic matter are found in the vicinity of river mouths and in the fine sediments of the Andaman Sea. The non-calcareous detrital fraction is dominated by quartz with minor amounts of orthoclase and plagioclase feldspars. The heavy mineral suite is complex because of the varied geology of the bordering land areas and consists primarily of leucoxene, ilmenite, staurolite, biotite and amphiboles. Kaolinite and mixed-layer minerals constitute the dominant clay minerals; lesser amounts of illite and montmorillonite also are present. A slight decrease in illite and an increase in mixed-layer minerals with depth was observed in some cores. Volcanic ash of an andesitic origin is found throughout much of the area. Many cores penetrate what appears to be a Late Pleistocene surface consisting of stiff, slightly indurated silty clay. The clay contains much peat, some of which has given a radiocarbon date of 10,000 years B. P.; it appears to represent a former tidal flat or estuarine deposit.