Oil palm residues are not currently suitable as feedstock for thermal energy generation because their high ash content can cause slagging, corrosion, and fouling. A water leaching treatment is a potential strategy to reduce the ash content in these residues. This study evaluates the effects of the duration and temperature of water leaching on two types of oil palm residues, namely, empty fruit bunches (EFBs) and palm kernel shells (PKSs). The optimum process duration for ash removal from EFBs was found to be 5 min, as the effect of convection on scrubbing was observed to remove substantial ash from the substrate during this period. A cross-flow model with estimated kinetic parameters of water leaching for EFB and PKS was developed and showed that three leaching stages of EFB achieved the greatest ash reduction from 5.47% to 2.63%. A low ash content of PKS showed no value for ash removal in any leaching process. Although there was no significance in the total ash reduction due to temperature effects, the leaching treatment was found to be most effective in reducing potassium, from 2.42% to 0.69% and 0.36% at 25 and 55 degrees C, respectively.