A great number of studies have been performed dealing with the use of ozone in the tertiary treatment of wastewaters in both laboratory and pilot plant scales. As each study had its own particular experimental conditions, the use of ozone is one of the only common parts of these studies. Some of the different results obtained may be explained upon examination of the phenomena. A standard pilot plant adapted to ozonation purposes is described so that the study of parameters for guide–lines for process automatization becomes easier. Generally, the choice of feed gas for the ozone generators has been determined on the basis of economics. However, it is interesting to note the influence of such gas not only on the ozone concentration but also on the classical wastewater parameters. Since comparisons are possible, a better understanding of the reactions and a better economical point of view are obtained in the treatment of wastewaters with ozone.