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A large part of CO2 emitted by human activities in the atmosphere is being absorbed by the oceans, leading to a progressive acidification, with detrimental effects on marine organisms, particularly those that build a skeleton of calcium carbonate. To better understand the magnitude of this environmental problem, it is necessary to know how seawater pH has oscillated in the past, at different timescales and locations. Because instrumental records of seawater pH exceeding a couple of decades are not yet available, past variations of pH need to be reconstructed using indirect indicators or proxies in suitable archives. A very promising geochemical proxy is the isotopic composition of boron in fossil biogenic carbonates which is, so far, the only practical method to quantitatively determine seawater pH variations back through time. In this communication, we present and discuss the theoretical grounds of this proxy and some of the paleo-pH reconstructions published so far.