The stable boron isotope composition (δ11B) of fossil foraminiferal shells is used as a paleo-pH recorder and is therefore one of the most promising paleocarbonate chemistry proxies ("paleoacidimetry"). One crucial question regarding this proxy is whether foraminifera record the pH of the bulk seawater or the pH of the microenvironment (diffusive boundary layer, ∼500 μm), which is strongly influenced by life processes. Here we present a novel theoretical approach to address this question by using a diffusion-reaction model. Model results indicate that the δ11B in planktonic foraminifera is primarily controlled by the pH of the microenvironment. We therefore predict that the δ11B of different species (e.g., symbiont-bearing versus symbiont-barren) or of foraminifera grown in the dark and in the light should be offset from the δ1B of inorganic calcite. This theoretical prediction was experimentally confirmed while this paper was written [Hönisch et al., 2003]. Most importantly, the model predicts that this offset is constant over a wide pH range. Thus the use of δ11B as a paleo-pH indicator is not compromised through vital effects as modeled here.