In this research, catalytic decomposition of hydrogen peroxide by granular size geothite (alpha-FeOOH) was investigated in aqueous medium that simulates natural waters. Carbonates and humic acids were used in the solutions to represent the natural organic and inorganic constituents of water resources, respectively. This research was emphasized on the decomposition rates of hydrogen peroxide that might be related to the removal rates of contaminants through the generation of highly reactive OH in water. These reactions are of interest for understanding the chemistry of natural waters that contain hydrogen peroxide and iron. The results demonstrated that geothite sucessfully catalyzed hydrogen peroxide decomposition in the solutions simulated to natural water resources. The decomposition of hydrogen peroxide followed first order reactions for the applied hydrogen peroxide dosages in the range of 1.47 to 8.82 mM. Decomposition rate was independent of the initial dosages of hydrogen. peroxide. On the other hand, decomposition rate was increased at relatively high concentrations of humic acids. Decomposition predominantly occurred on the solid phase and the amount of iron dissolved from geothite was very low in concentration to have a significant effect on hydrogen peroxide decomposition in the presence of humic acids and carbonates.