Sugars, prominently fructose, have been shown to accelerate the degradation of food components during thermal treatment. Yet, the mechanism by which this occurs is not well understood. Fructose and glucose have been reported to undergo autoxidation to generate reactive oxidative species (ROS) under physiological conditions; however, information on ROS generation during thermal treatment is limited. We observed that hydrogen peroxide was generated during thermal treatment (up to 70 °C) of aqueous solutions of fructose and glucose (up to 10% w/v), with significantly higher concentrations observed in fructose solutions. The rate of generation of hydrogen peroxide increased with temperature, pH, oxygen concentration and the presence of phosphate buffer. Singlet oxygen was also detected in fructose and glucose solutions prepared in phosphate buffer. Results of this study indicated that fructose and glucose undergo oxidation during thermal treatment resulting in generation of ROS that may have deleterious effects on food components.