Fossil foraminiferal shells are utilized in paleoceanography to extract information about environmental conditions of the past ocean. Based on several assumptions, the ratio of 13C and 12C preserved in their shells is used to reconstruct, for example, the paleoproductivity or the oceanic pCO2. Metabolism of the living organism and the sea water chemistry, however, can influence the incorporation of carbon isotopes during calcification such that the signal of the shells differ from the signal of the sea water. These effects occur because the chemical microenvironment of the foraminifer (boundary layer thickness ~ 500 μm) differs from the bulk sea water. Here, we present a numerical model that calculates the δ13C of the foraminiferal shell as a function of the sea water chemistry and the magnitude of vital effects. Concentration profiles of the chemical species of the carbonate system within the microenvironment of foraminifera are obtained by solving diffusion-reaction equations. The compounds of dissolved forms of carbon dioxide containing either the stable carbon isotope 13C or 12C are considered separately. Spherical symmetry of the foraminifer is assumed. The model outcome is compared to results from culture experiments with the planktonic foraminifer Orbulina universa. Model results indicate that the interaction between vital effects of the foraminifer and the sea water chemistry can account for changes in the δ13C of foraminiferal calcite of 0.3-0.4‰ when glacial and interglacial sea water conditions are compared. These effects occur even though the δ13C of the total dissolved inorganic carbon is kept constant. Thus, changes in sea water chemistry should be distinguished from events which changed the δ13C of the inorganic carbon of the sea water.