Antioxidant is one of the important additives to be used to increase the oxidation stability of biodiesel. The activity of antioxidants can be increased by the addition of a binary antioxidant mixture. In some cases, binary antioxidants are used to significantly increase the performance of antioxidant. Binary antioxidant mixtures have been shown to have a synergistic effect to improve the oxidation stability of biodiesel. The synergistic effect produced between two antioxidants is reported primarily as a result from the formation of heterodimers between antioxidants during the autoxidation process. The heterodimers have more hydroxyl groups. Therefore, it has better antioxidant activity than its main antioxidant in a single component. The previous study proposed the chemical structure of heterodimer but has not been proven. This study observed reactions of binary antioxidant mixtures of Pyrogallol and Ter-Butyl Hydroquinone in methanol solution with UV light exposure to initiate and accelerate the occurrence of the autoxidation process. The samples were qualitative and quantitatively evaluated to identify the heterodimer product. In addition, the effect of the antioxidant ratio in the binary mixtures was also evaluated on the reaction product and the synergistic effects of heterodimer products. The optimum ratio of binary antioxidant mixture to biodiesel was also determined to obtain the best concentration. From three combination of Pyrogallol and Ter-Butyl Hydroquinone, 1:1, 1:3, and 3:1, 1:1 ratio showed best synergistic effect and highest induction period from 500ppm antioxidant addition to palm oil biodiesel. The result obtained from FTIR and LCMS showed a promising result of heterodimer existence that support previous study. FTIR instrument detected new peak of ether bond and LCMS instrument detected the existence of heterodimer with molecular mass 290 amu.