Abstract: Abstract: Conjugated linoleic acid (CLA) is one of the unsaturated fatty acids that have multiple bioactivities. It is liable to oxidation, particularly when exposed to light, heat and oxygen. Encapsulation of CLA in O/W emulsions stabilized by hydrocolloids could improve the oxidative stability of CLA, and could extend its applications in foods. This paper investigated the physical and chemical stabilities of O/W emulsions prepared with two CLAs of different viscosities and stabilized with modified gum arabic EM2. The O/W emulsions consisted of 15% CLA and 5%-25% EM2, and were prepared by pre-homogenization using a high-speed blender followed by one pass through a high-pressure homogenizer at 75 Mpa. The physical stability of CLA emulsions was evaluated by measuring the change of particle size and size distribution during storage at room temperature (25 oC) and at an elevated temperature (60 oC), whilst the chemical stability was characterized by the formation of lipid oxidation products. The concentration of primary lipid oxidation products was measured using the peroxide value (PV) method and the concentration of secondary oxidation products measured using the anisidine value (AV) assay. The results show that higher EM2 concentrations lead to finer and more homogenous emulsions. CLA oxidative stability is dependent on the physical stability of emulsions. For the CLA with lower viscosity, at all EM2 concentrations investigated, the emulsions show good physical stability with no significant changes in particle size distribution within seven days both at 25oC and 60oC. The oxidative stability correlates positively with EM2 concentration. For the other CLA with higher viscosity, excessive concentration of EM2 could reduce physical stability and thus decrease oxidative stability of CLA emulsions. 5% EM2 was found to be the optimal concentration stabilizing CLA emulsions. The reduced physical stability of CLA emulsions at higher EM2 concentrations could be due to the depletion force effect caused by the excess of free EM2 molecules. The reduced oxidative stability could also be attributed to the higher concentrations of intrinsic prooxidative metals, i.e. iron and possibly hydroperoxides as well, contained in EM2 solutions of higher concentrations.