A method for increasing ozone concentration in a liquid can include: providing a gas having ozone; introducing the ozone-containing gas into a liquid, wherein the liquid and ozone combination has a temperature between about 0.8 and about 1.5 times the critical temperature of ozone; and increasing isothermally, the pressure of the ozone-containing gas above the liquid to about 0.3 to about 5 times the critical pressure of ozone so as to increase the ozone concentration in the liquid. The temperature is expressed in absolute units (Kelvin or Rankin). The method can be used for removing ozone from a gas or for purifying ozone. The liquid having a high ozone concentration can be used for ozonolysis of a substrate.