Chemistry. A strong oxidizing agent used in aqueous solution as a bleach and disinfectant. It dissociates into water and molecular oxygen. In animal and plant tissues, it is built by enzymes (e.g., oxidases) during oxidation of amino acids, fatty acids and the degradation of carbohydrates and nucleic acids. Hydrogen peroxide is cytotoxic and decomposed by the enzymes catalase and peroxidase in the cell.