The objective of the research described in this paper was the development of a fully mechanistic model for ozone mass transfer from the gaseous to aqueous phase and aqueous ozone self-decomposition in a continuous-flow countercurrent ozone contactor. The developed model incorporated concepts describing the rate and extent of ozone mass transfer from gaseous phase to aqueous phase and comprehensive consideration of ozone decomposition reactions in the aqueous phase. Simulation of the effects of changes in the influent gaseous ozone concentration, aqueous-phase pH, and scavenger concentration on the ozone mass transfer and decomposition was accompanied by an explanation of the simulation results based on the current understanding of ozone mass transfer and aqueous ozone chemistry. Experimental work included fabrication and operation of a laboratory-scale continuous-flow countercurrent ozone contactor. Experimental data concerning the steady-state aqueous ozone concentration in the reactor and the effluent gaseous ozone concentration from the reactor were obtained at various influent gaseous ozone concentrations, aqueous-phase pHs, and aqueous scavenger concentrations. The experimental data were then compared with the results obtained from simulation runs conducted under similar conditions. A comparison of experimental data and model simulation results under similar conditions showed an adequate positive correlation. The discrepancy between some experimental data and simulation results was discussed in terms of the sensitivity of the ozone mass transfer and aqueous-phase self-decomposition to changes in the temperature and ionic strength of the aqueous medium.