Sonication was carried out under atmosphere of air, oxygen, nitrogen, argon, or their gas mixtures. The value of pH in water for different dissolved gases was obtained under a constant ultrasonic intensity. The ultrasonic intensity in the reaction vessel was measured by use of the decomposition reaction of porphyrin in water. Under an air atmosphere, the pH value changed from 7 to 4.3 for an irradiation time of 30 min and after that the pH value approached to 3.7. On the other hand, the pH values for water saturated with Ar, Nz or Oz slightly decreased. In the mixture of Ar, Nz and Oz, the minimum pH in water saturated with the Ar+O-2 mixture was observed around 60 % of Ar, and that saturated with the Ar+ (N-2 and O-2) mixture at around 30 % of Ar. The amount of OH radical produced on sonication was consistent with that estimated from products obtained by decomposition of water. From these results, it is concluded that the decrease of pH is influenced not only by N-2, but also by O-2.