Mass transfer of ozone and oxygen to water was investigated both in pilot plant countercurrent bubble column and in a Rushton type laboratory stirred reactor supplied with a variable speed turbine agitator. A comparison was made for different hydrodynamic conditions with the main task of developing an engineering approach for determination of the physical volumetric mass transfer coefficient (KLoa), specific interfacial area (a), and physical masstransfer coefficient (KLO). The mass transfer characteristics of ozone and oxygen can be determined quickly in a pilot plant or laboratory apparatus, and employed to optimize the performance ofthe full scale water treatment plant.