Primary and early diagenetic carbonate minerals may be associated with organic carbon-rich lacustrine sediments. The stable oxygen and carbon isotopic compositions of these carbonates can yield useful paleoenvironmental information. Primary carbonates from hydrologically open lakes typically show little or only poorly correlated covariance between oxygen and carbon isotopic variations, whereas carbonates precipitated from surface waters of closed lakes display characteristic, highly correlated covariance. Oxygen isotopic ratios in early diagenetic phases reflect the isotopic composition of the waters in which the host sediments were deposited. The carbon isotopic composition of diagenetic carbonates is determined by the nature of the dominant microbiological processes inv lved in diagenesis of organic matter during the earliest stages of burial. In lakes poor in dissolved sulfate, bacterial methanogenesis is dominant and produces carbonates with markedly positive ¹³C values. Sulfate-rich lakes favor bacterial sulfate reduction that leads to precipitation of carbonates with negative ¹³C signatures. Using isotopic data from carbonates from Lake Bosumtwi, Ghana, as a case study, we show that characteristic relationships exist between isotopic compositions of primary and diagenetic carbonates of sulfate-poor and sulfate-rich lakes.