The oxygen isotope compositions of fossil biocalcites, such as foraminifera, bivalves, brachiopods, and belemnites have allowed for reconstructions of sea surface and deep ocean temperatures throughout the Phanerozoic and constitute the most important record of paleo-ocean conditions. However, following the death and burial of these marine organisms, their shells encountered porewaters with isotope compositions and temperatures different from the seawater in which they lived. This could have driven post-burial isotope exchange that would significantly alter the original biocalcite O-isotope compositions and hence introduce bias in paleo-seawater temperature reconstructions. These processes need to be better understood to improve the accuracy of marine paleo-environment reconstructions, which are essential to contextualize anthropogenic climate change in a geological perspective. In this thesis, the calcitic tests of three species of modern benthic foraminifera (Ammonia confertitesta, Haynesina germanica, and Amphistegina lessonii) and one fossil benthic foraminifera (Ammonia beccarii), as well as the calcitic prismatic layers of two modern bivalve mollusks (Pinna nobilis and Pinctada margaritifera) were subjected to experiments simulating the effects of fluid-mediated isotope exchange. Tests and shells were incubated in a highly 18O-enriched artificial seawater at 90 °C for 6 days, at chemical equilibrium. The experimentally incubated samples were texturally indistinguishable from pristine specimens but their bulk oxygen isotope compositions indicated rapid and substantial isotope exchange with the artificial seawater. NanoSIMS imaging, combined with scanning electron microscopy, electron backscatter diffraction, and cathodoluminescence was used to visualize and quantify how fluids penetrated and isotopically exchanged with these biocalcites. The analyses revealed that oxygen isotope exchange was closely linked to species-specific differences in shell and test ultrastructures and the distribution of intracrystalline organic matter. Comparison between the extent of isotope exchange in modern benthic foraminifera tests (Ammonia confertitesta) and fossil equivalents (Ammonia beccarii) showed that when the intracrystalline organic material is already partially degraded by the natural fossilization process, the rate of diagenetic O-isotope exchange is lower than in the pristine tests, but O-isotope exchange nevertheless remains an effective process. Fossil biocalcites thus remain susceptible to O-isotope exchange millions of years after sedimentation and burial. Correlated NanoSIMS imaging and Photo-induced Force Microscopy mapping of the calcitic prismatic layer of bivalves revealed that the distribution of intracrystalline saccharide-rich organic matter was closely linked to the observed distribution of O-isotope exchange, while intracrystalline proteinaceous material was generally scarce. This suggests that intracrystalline saccharides play a much more prominent role in facilitating diagenetic isotope exchange than intracrystalline proteins. By identifying a species-specific susceptibility to diagenesis and elucidating the role of intracrystalline organic matter during O-isotope exchange in foraminifera tests and prismatic bivalve layers, this thesis contributes to our understanding of diagenesis in biocalcites and represents a significant step towards the development of more accurate reconstructions of paleo-ocean conditions.