In this note, we review and explain which key parameters of composition of awater are required for quantifying its physical and chemical properties withrespect to ozonation reactions. The quality of the water, however, often has to be characterized beyond such analytical data. Additional parametersinclude an operationally defined rate with which added ozone is depleted (seePart II, following this paper) and by the parameters which characterize theconsumption of OH radicals produced as secondary oxidants when ozonedecomposes (see Part III, in preparation). The present series of notes shouldstimulate discussions on the selection of a minimum set of information andlead to guidelines on data requirements for ozonation processes.