Proposed uses of ozone for stand-alone cooling water treatment raise critical questions as to what happens chemically. These questions are of more significance to industrial cooling water systems, which typically have higher temperatures and cooling ranges than do comfort cooling systems. When applying ozone to cooling waters, it is very important for the user to understand many fundamental aspects of ozone chemistry. For example, when ozone is added at water pH levels often encountered in cooling waters (≥ 8), it decomposes to form hydroxyl free radicals, which are stronger oxidizing agents than molecular ozone itself, but of microsecond half-life, and therefore are poor disinfectants. The presence of bicarbonate alkalinity, hardness, naturally occurring organics, bromide ion, and effects of pH levels on water quality parameters and molecular ozone, have pronounced effects on chemical reactions which occur when ozone is added to cooling waters. The authors review the fundamental chemistries involved with ozone in water, discuss the effects of water constituents present or expected to be present in recycling cooling waters, relate these aspects to biocidal efficacy of ozone treatment, and explore possible mechanisms for scale and corrosion control in cooling systems by ozone.