A method for the potentiometric titration of secondary phosphate groups in nucleic acids is described. Ribonucleic acids of yeast and of microsomes contain 5—6% secondary phosphate groups which cannot be removed by dialysis. The potentiometric method was applied to study several enzymatic hydrolyses and the non-enzymatic hydrolysis between pH 2.4 —1.8. The rate of hydrolysis for the purines and for the phosphate groups is approximately proportional to the H®-concentration. The constants of hydrolysis for ribonucleic acid and for deoxyribonucleic acid were determined. In DNA the depurinisation is 650 times faster than in RNA.