Cooling water treatment generally is made with the addition of chlorine, although it´s application has some disadvantages. There is an active development in Brazil and other countries to use alternative chemical disinfectants in place of chlorine, such as hydrogen peroxide, a powerful oxidant which is known for its high biocidal efficiency. The aim of this research is to study the effectiveness of hydrogen peroxide as a disinfectant compared to chlorine in different operational conditions. The experiments were carried out using an water sample from a cooling water system of a steelmaking plant in the city of Rio de Janeiro. The microbial contamination of this water sample was measured without adding any kind of disinfectant. After that, water sample was treated by adding hydrogen peroxide and chlorine, in order to compare and evaluate the efficiency of the two biocides to control bacterial growth in water. Besides microbiological tests, experiments were conducted to compare the degree of corrosion caused by the addition of hydrogen peroxide and chlorine in water. The experimental methodology employed 1020 carbon steel specimens and corrosion rates were measured by weight loss determination after the period of exposure. The results showed that the application of hydrogen peroxide leads to satisfactory bacterial control. However, compared to chlorine, hydrogen peroxide is a rather poor disinfectant. The efficiency of hydrogen peroxide depends on reaction time and it is affected by dissolved polluants in water. Evaluation of corrosion rates showed that hydrogen peroxide causes basically the same corrosion rates than chlorine.