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Coralline algae provide important ecosystem services but are susceptible to the impacts of ocean acidification. However, the mechanisms are uncertain, and the magnitude is species specific. Here, we assess whether species-specific responses to ocean acidification of coralline algae are related to differences in pH at the site of calcification within the calcifying fluid/medium (pHcf) using delta B-11 as a proxy. Declines in delta B-11 for all three species are consistent with shifts in delta B-11 expected if B(OH)(4)(-) was incorporated during precipitation. In particular, the delta B-11 ratio in Amphiroa anceps was too low to allow for reasonable pHcf values if B(OH)(3) rather than B (OH)(4)(-) was directly incorporated from the calcifying fluid. This points towards delta B-11 being a reliable proxy for pHcf for coralline algal calcite and that if B(OH)(3) is present in detectable proportions, it can be attributed to secondary postincorporation transformation of B(OH)(4)(-). We thus show that pHcf is elevated during calcification and that the extent is species specific. The net calcification of two species of coralline algae (Sporolithon durum, and Amphiroa anceps) declined under elevated CO2, as did their pHcf. Neogoniolithon sp. had the highest pHcf, and most constant calcification rates, with the decrease in pHcf being 1/4 that of seawater pH in the treatments, demonstrating a control of coralline algae on carbonate chemistry at their site of calcification. The discovery that coralline algae upregulate pHcf under ocean acidification is physiologically important and should be included in future models involving calcification.