pH measurements with a glass electrode at 1000 kg cm−2 give direct evidence of the pressure induced ionization constant shift and permit calculation of the corresponding volume change for the reaction . Extrapolation at zero ionic strength and zero acid concentration is used to determine log for formic acid, phosphoric acid, and phosphate buffer (step 1). Data concerning phosphate buffer (step 2), acetic acid, acetate buffer, carbonic acid, bicarbonate buffer, sodium bicarbonate, sodium acetate, and ammonia are given together with the results of direct pH measurements at great ocean depth (2350m). Work done in the same field but based on conductivity determinations is briefly reviewed.