The first comprehensive study - bench-top laboratory investigations, pilot scale testing, and critical monitoring and evaluation of field applications - addressing the effects of ozone as a stand-alone cooling water treatment program is presented. The study also represents the first critical comparison of ozone-treated systems with non-treated systems. Excellent corrosion control can be attained in ozone-treated cooling water systems. However, the corrosion rates are completely dominated by the water chemistry of the system and have no dependence on the presence of ozone at typical use levels. Good control of fouling can also be attained. However, as was the case with corrosion control, deposition on the heat exchange surfaces is not determined by the presence of ozone, but by several factors that traditionally influence fouling in a system. The strong biocidal properties of ozone resulted in excellent microbiological control in all PCT investigations, and in both case studies. Excellent agreement was observed among all stages of testing.