The lifetime value of ozone in a water is one of the key parameters for describing the characteristics of a water considering an ozonation process and for planning, optimizing, and steadily adapting the ozonation process. This lifetime is highly sensitive with respect to the chemical composition of the water. Interferences of different radical-type chain reactions make it difficultto predict it solely from analytical characterizations. The waters with respect to the lifetime of ozone, therefore, have to be characterized by direct laboratory experiments. However, because the half-life times depend on the type of operation used for the measurement, it is appropriate to formulate corresponding standardized guidelines that allow comparison whenever the responses of different waters have to be compared, and whenever characterizations have to be followed during the decades during which a water utility applies a process.