The signal detection of quantitative 1 H nuclear magnetic resonance ( 1 H qNMR) for the amount of water in HCl and KOH aqueous solutions at low pH (pH −1.2) and high pH (pH 16) was discussed, and an "anomaly of decreasing 1 H NMR signal intensity" was observed. After adding significant amounts of acid (H + ) or base (OH − ) to a solution for pH regulation, the number of 1 H nuclei observed by 1 H NMR significantly decreased at pH ≤ 2 and ≥ 13. The mobility and the activity coefficient of some or all the water molecules that are hydrated to H3O + significantly decreased; hence, they could not be detected by 1 H qNMR. The remarkably strong electrostatic interactions of the solvent with H3O + and OHsignificantly reduce the activity coefficient of the solvent by restricting the solvent molecule to the vicinity of the ion.These results are comparable to those reported for vapor pressure measurements and can be attributed to a decrease in activity. H + (H3O + ), which has a relatively small ionic radius, has a significant effect on the solvation structure and hydrogen-bond network owing to strong electrostatic interactions with the solvent. Subsequently, significant reduction in the 1 H NMR relaxation time of the water molecules and signal intensity, along with a low magnetic field shift in the 1 H NMR signal were also observed in the strongly acidic and strongly basic regions.