RECORDS Of past changes in the pH of the oceans should provide insights into how the carbonate chemistry of the oceans has changed over time. The latter is related to changes in the atmospheric CO2 content, such as that which occurred during the last glacial-interglacial transition(1). Previous studies(2,3) have shown that the fractionation of boron isotopes between sea water and precipitated carbonate minerals is pH-dependent. This finding has been used to reconstruct the evolution of ocean pH over the past 20 million years by analyses of boron isotopes in the carbonate shells of foraminifera(4). Here we use the same approach to estimate changes in ocean pH between the last glacial and the Holocene period. We estimate that the deep Atlantic and Pacific oceans had a pH 0.3 +/- 0.1 units higher during the last glaciation. The accompanying change in carbonate ion concentration is sufficient to account for the decrease in atmospheric p(co2) during the glacial period(1). These results are consistent with the hypothesis(5) that the low CO2 content of the glacial atmosphere was caused by an increased ratio of organic carbon to carbonate in the ’rain’ to the sea floor, which led to an increase in carbonate ion concentration (and thus in pH) of deep water without a corresponding increase in the lysocline depth.