Changes in the Sr to Ca ratio of sea water have important implications for the interpretation of past climate. It has proven difficult to interpret Sr/Ca of foraminiferal calcite as a measure of seawater Sr/Ca or as reflecting the influence of deep water carbonate ion saturation (Delta[CO32-]) on the incorporation of Sr into benthic foraminiferal carbonate. Here, we address this issue by measurements of paired benthic foraminiferal Sr/Ca and B/Ca (a proxy for deep water A[Delta[CO32-]) for core-tops from the global ocean and three down cores at different settings during the Last Glacial-interglacial cycle. These new data suggest a significant control of deep water Delta[CO32-] on benthic foraminiferal Sr/Ca, and that down-core shell Sr/Ca variations can be largely accounted for by past deep water Delta[CO32-] changes. We conclude that seawater Sr/Ca has likely remained near-constant on glacial-interglacial timescales during the late Pleistocene, in agreement with model results. With due caution, benthic Sr/Ca may be used as an auxiliary proxy for deep water Delta[CO32-] if seawater Sr/Ca is constant.