peer reviewed ; Carbon isotopic variations of Frasnian shallow-water carbonates from Belgium are related to facies and major sea-level trends. The influence of the diagenetic overprint was assessed in order to determine the primary signal of the Frasnian carbonates. Shallow-water microfacies are characterized by biostromes with stromatoporoids and lagoonal deposits dominated by carbonate mud and calcareous algae with subaerial exposure surfaces. The diagenetic history was controlled by three main events: early meteoric diagenesis (short-term subaerial exposure during deposition), late meteoric diagenesis (major Famennian regression) and burial diagenesis. The oxygen isotopic values are almost constant with respect to facies, original material (carbonate mud and cement) and sedimentological units (no differences before or after the main regression). This homogeneity is related to resetting during late meteoric diagenesis. The carbon isotopic values are related to facies (with the more negative values for the shallowest facies) and to major sea-level variations (most negative values after the main regression). This pattern is interpreted as being related to primary signals. This trend was enhanced by early meteoric diagenesis and the influence of more negative values from paleosols. The carbon isotope patterns reflect the influence of sea-level and water circulation on shallow water deposits and this influence implies that shallow-water carbonates are not necessarily good material for assessing the primary isotopic values of the ocean because of the influence of long residence time (“aging”) of the platform-top water and because of early diagenesis.