Paleo-pH reconstructions based on boron isotopic composition of foraminifera have been used to estimate glacial--interglacial changes in surface ocean pCO2 of the northwest African upwelling zone. On comparison with a similar study for the eastern equatorial Pacific upwelling zone, it can be concluded that the two major upwelling zones acted quite differently during the glacial periods as compared to today. While the pH of the surface ocean off northwest Africa was 0.2¿0.07 units higher during the glacial period compared to that during Holocene, there was no significant glacial--interglacial change in the surface ocean pH in the eastern equatorial Pacific. Carbonate chemistry reconstructions based on the estimated pH changes suggest that the ocean-atmosphere pCO2 gradient off northwest Africa was lower by at least 70¿40 μatm during glacial periods compared to during the Holocene. In contrast, the ocean-atmosphere pCO2 gradient in the eastern equatorial Pacific was higher by at least 80¿40 μatm during glacial periods as compared to during the Holocene. Hence the eastern equatorial Pacific upwelling system was a significantly larger source of CO2 to the atmosphere, while the one off northwest Africa was a significantly smaller source of CO2 during the last glacial period. The pCO2 reconstructions further indicate that in spite of higher glacial productivity compared to during the Holocene, neither of the two areas became a sink of CO2. ¿ 1999 American Geophysical Union