It has been shown that the acidity of natural water increases upon cooling to the room temperature after being boiled. We have pointed out the influence of heat on the acidity of natural water and found that boiling shifted the pH to the acidic side. Further exact treatments of these properties in metal salt solutions were attempted and the depression of pH of salt solutions before and after boiling was determined. The following metal salt solutions showed a marked depression of pH: FeSO4·7H2O(1.8), AlCl3.6H2O(1.1), KAl(SO4)2.H2O(1.0), CuSO4.5H2O(0.7), CaCl2(0.3). However the salts of Co, Ni, Mn showed no influence. The depression of pH is accelerated by the anions, such as phosphate and silicate, but fundamentally, it is due to the characters of the metal ion itself. This mechanism may be explained as follows; [Cu(H2O)4]++→[Cu(OH)(H2O)3]++H+ The depressed pH does not return generally to the original value for a few days, except in CuSO4 solution. For the determination of pH of hot springs which are changed by the history of heating, we must investigate the depression of PH through boiling as well as the course of it after drawing. We must be careful in the sterilization of culture medium or buffer solution containing FeII salts because the growth of bacteria is checked owing to the increased acidity of the media.