Ozone decay kinetics in various natural waters and their impact on modeling dissolved ozone concentration profiles in a bubble column contactor were studied. The results demonstrated that the commonly-used first-order kinetics with a constant rate constant were unable to describe the ozone decay behavior, particularly in cases where the initial ozone demand was significant. An explanation is that the water's reactivity toward ozone changes substantially as the ozonation depletes those readily reactive compounds. As a result, an exponentially decreasing function was proposed to provide better estimates of the ozone decay curves.