Field and laboratory observations have established the occurrence of diurnal cycles in pH, dissolved CO 2 , and carbonate saturation of natural marine waters in a wide range of tropical and sub-tropical environments. The cycles are believed to result from changing rates of the opposing reactions respiration and photosynthesis. During daylight hours, photosynthesis is the dominant reaction, causing a reduction of dissolved CO 2 to a late afternoon minimum, accompanied by a gradual increase in pH to a late afternoon maximum. At night, respiration is unopposed by any reaction which significantly reduces the activity of dissolved CO 2 , and the resulting build-up reaches a maximum, with an accompanying pH minimum, shortly before dawn. Well mixed surface waters exhibit diurnal pH variations in excess of 0.15 pH unit, and changes in carbonate saturation greater than 8 millivolts. In restricted water bodies the diurnal changes are substantially greater. In carbonate environments, the increased activity of dissolved CO 2 , during the hours of darkness favors solution of solid carbonate phases in contact with the waters, and may lead to chemical erosion of consolidated carbonate rocks. During daylight, photosynthetic extraction of CO 2 reduces tee solubility of carbonate phases, and leads to supersaturation of the affected waters. In the areas studied, the supersaturation of surface waters with respect to calcium carbonate is shown to be fully explicable in terms of a metastable equilibrium between the waters and the most soluble carbonate phase with which they are in contact at the time of maximum activity of dissolved CO 2 . The amplitude of the daily change in carbonate saturation equals or exceeds the lateral variations reported in earlier synoptic studies, and therefore casts doubt upon the validity of the synoptic studies now available.