This chapter presents the results of hydrographic and chemical studies performed in April 1981 and July 1983 in the South China Sea near the mouth of the Mekong River. Distribution of salinity, dissolved oxygen, and carbonate parameters (alkalinity, pH, total inorganic carbon, partial pressure of carbon dioxide, and degree of water saturation with calcium carbonate) during dry and rainy seasons were studied. Studies showed that during the rainy season (July 1983), in contrast to the dry season (April 1981), the coastal surface waters were generally characterized by lower temperature, salinity, alkalinity, total inorganic carbon, dissolved oxygen and higher AT/Cl, CT/Cl and carbon dioxide partial pressure values. The diurnal variability of the carbonate system in both dry and rainy seasons is mainly due to changes in the ratio of the intensities of photosynthesis and decomposition of organic matter. The processes of simple mixing of river and sea water are essential for surface horizons only in the phase of low-water tide.