A corporate research center has successfully operated 2 separate ozonation systems on HVAC cooling towers at comparatively high cycles of concentration since December of 1987. Each system uses ozone as the sole method of water treatmert and employs no other chemical or mechanical treatments. This paper describes the HVAC systems, the preexisting conditions at the facility prior to ozonation, the specific details of the ozonation systems, and the excellent functional performance and results achieved through ozonation to date. Operational shortcomings which were experienced are also described as well as the corrective measures taken to alleviate the problems.