Activation process and active site of Co-based Ziegler-Natta catalysts were studied by using synchrotron X-ray absorption spectroscopic technique and an optimum model of the cobalt active site was proposed with density functional theory (DFT) calculations. In the X-ray absorption near edge structure (XANES) spectrum of Co(2-ethylhexanoate)(2), the preedge peak, 1s --> 3d transition at 7707.8 eV and the K-edge peak, 1s --> 4p transition at 7719.2 eV were observed. Both the intense K-edge and the weak preedge peaks indicate that Co(2-ethylhexanoate)2 possesses high OF, symmetry and the cobalt exists in a divalent state. Upon addition of methylaluminoxane (MAO), some changes in peak positions and intensities were observed. The preedge peak was a little shifted to 7708.2 eV with increased intensity, and the K-edge peak was shifted to a lower energy, 7717.2 eV with decreased intensity. This indicates that the oxidation state of cobalt still mainly remained +2 and the coordination geometry was altered from O-h to T-d resulting from Co 3d and 4p orbital mixing with a loss of O-h symmetry. In the XANES spectrum of a mixture of Co(2-ethylhexanoate)(2), MAO and t-BuCl, the K-edge peak at 7715.7 eV was observed. The orbital mixing of 4p cobalt and 2p chlorine orbitals results in a low energy transition. In 1,3-butadiene polymerization, this is attributable to the coordination of chlorine to cobalt, which is believed to facilitate cis-configuration and increasing reactivity. B(C6F5)(3) was employed as Lewis acid, but no significant interaction with cobalt was found in the XANES spectrum of a mixture of Co(2-ethylhexanoate),, Al(iBu)(3), and B(C6F5)(3), which is similar to the XANES of a mixture of Co(2-ethylhexanoate)(2) and Al(iBu)(3). B(C6F5)(3) as Lewis acid does not greatly disturb the coordination symmetry of cobalt nor influence the oxidation state. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2186-2190, 2009