Abstract: L,L-ethylene dicysteine(L,L-EC) is a diaminodithiol(N₂S₂) ligand used in nuclear medicine for the preparation of ^99mTc-L,L-EC, a tracer agent for renal function studies. The dithiol groups of L,L-EC are easily oxidized and form the corresponding disulfide. In this paper, the dynamic oxidation process of L,L-EC was studied by ultra-performance liquid chromatography tandem mass spectrometry(UPLC/MS/MS) in multiple reaction monitoring(MRM) mode. The kinetic experiments were conducted at three different temperatures. The reaction order and the rate constants for the transformation of L,L-EC were calculated using kinetic data under pseudo-first-order reaction conditions. The activation energy of the oxidation of L,L-EC was calculated by an Arrhenius graph.