Biodiesel, an alternative fuel derived from lipid feedstocks, such as vegetable oil or animal fat, is mainly composed of saturated and unsaturated fatty acid alkyl esters. Fuel suppliers, terminal operators, and users are becoming more concerned with monitoring and maintaining good biodiesel fuel quality with respect to oxidative degradation during storage. The oil stability index (OSI), a parameter that measures the relative oxidative stability of fatty materials, is typically measured isothermally at elevated temperatures to accelerate oxidation. The present work investigates the effects of block temperature (T) on the OSI of biodiesel from soybean oil fatty acid methyl esters (SME) and used cooking oil fatty acid methyl esters (UCOME). Results were compared to those for pure methyl oleate (MO). An increasing temperature accelerated the oxidation reaction causing a decrease in OSI. Response factors (RF) determined with MO as the reference methyl ester showed little variation with respect to the temperature ranges studied. At constant T, SME yielded lower OSI values than either UCOME or MO. However, despite having comparable iodine values, UCOME yielded a significantly higher OSI than MO. Two mathematical models for determining OSI as a function of T demonstrated linear correlations for all, three methyl esters. Results from the second model, ln(OSI) versus T-1, were employed to calculate the activation energy (E-a) of first-order oxidation reactions. Although both models exhibited relatively small deviations between calculated and measured OSI values, the models were not reliable at temperatures below 50 degrees C.