The transesterification of palm oil to methyl esters (biodiesel) was studied using KOH loaded on Al2O3 and NaY zeolite supports as heterogeneous Catalysts. Reaction parameters such as reaction time, wt% KOH loading, molar ratio of oil to methanol, and amount of catalyst were optimized for the production of biodiesel. The 25 wt% KOH/Al2O3 and 10 wt% KOH/NaY catalysts are Suggested here to be the best formula due to their biodiesel yield of 91.07% at temperatures below 70 degrees C within 2-3 h at a 1:15 molar ratio of palm oil to methanol and a catalyst amount of 3-6 wt%. The leaching of potassium species in both spent catalysts was observed. The amount of leached potassium species of the KOH/Al2O3 was somewhat higher compared to that of the KOH/NaY catalyst. The prepared catalysts were characterized by using several techniques Such as XRD, BET, TPD, and XRF. (c) 2008 Elsevier Ltd. All rights reserved.