Abstract The purpose of this paper is to perform an analysis on cooling systems with ozonation water treatment equipment according to the second law (exergetic analysis) of thermodynamics. In addition, an analysis of ozonation systems themselves is made with different cooling alternatives. The exergetic analysis is better than just the energetic one (the first law of thermodynamics) because it gives a clearer explanation of the performance. In particular, it takes into consideration the impact of the system on the environment. According to the analysis in this paper, for the evaporative condenser, the minimum change in exergy with respect to the entering temperatures of air and water does exist. For other variables, the changes in exergy just increase or decrease monotonically with respect to mass flow rate, power input or temperature. For the cooling tower system investigated, a minimum change in exergy with respect to the entering temperatures of air and water does not exist. The study done by Carollo Engineers and the Metropolitan Water District of Southern California (MWD) was from an economic viewpoint, and from their report, Alternative (2A) was the most superior. However, from an exergetic viewpoint, Alternatives 1 and 5 are lower in the change of exergy than the other alternatives. Therefore, the exergetic analysis has been demonstrated to be useful for investigating cooling systems with ozonation water treatment.