One of important indicators of reliable operation of the steam-water cycle at power plants is water chemistry. Special chemicals are used to adjust properties of working fluids. At designing a new unit the chemicals are selected on the basis of parameters of water, steam and materials, used in different parts of the cycle.Another indicator is minimizing the cooling water loss. Typical loss of cooling water is about 5% of total water amount. This loss is associated with evaporation, droplet carry-over, and blowdown in cooling towers.For power plants, it is typically recommended to use dry cooling towers for minimizing the cooling water loss. In dry cooling systems, cooling water with low amount of impurities goes to mixing condenser, where turbine steam condensation occurs. Then one portion of mixed water flow at the condenser outlet is supplied to the boiler or steam generator feed water loop. Another portion is returned back to the cooling tower. Inside the cooling tower, water is cooled down by passing through air-cooled heat exchangers manufactured of aluminium or steel alloys.On the basis of Pourbaix diagram it is proposed to evaluate the influence of water chemistry on the forms of existence of aluminium compounds in dry cooling towers. The results of calculations are compared with the experimental data.