CexCu1-xO1.9 (x = 0.3, 0.5, 0.8 and 0.9) catalysts were synthesized by solid state method, using ball mill apparatus, and evaluated in medium temperature shift (MTS), as well as oxygen assisted MTS (OMTS) reactions at temperature range of 300-390 degrees C. Catalysts were characterized by X-Ray Diffraction (XRD), Temperature Programmed Reduction (TPR), Scanning Electron Microscopy (SEM), and N-2 adsorption-desorption (BET) analysis. Ce0.9Cu0.1O1.9 sample showed the best catalytic activity and structural properties. Decrease in proportion of Cu to Ce, leads to increase in the Cu-Ce mixed oxide formation (according to TPR analysis), significant increase in BET surface area (from 18 m(2)/g for Ce0.3Cu0.7O1.9 -48 m(2)/g for Ce0.9Cu0.1O1.9), and decrease in CuO crystalline size (XRD). Moreover, the effect of oxygen addition to the feed (O-2/CO ratio = 0, 0.3, 0.5, 0.7 and 1), on the catalytic performance was evaluated. CO conversion was increased by enhancing the amount of oxygen (from 60% to 80% at 360 degrees C). CO and H-2 oxidations are occurring as competitive parallel reactions in which CO oxidation is dominant at low O-2/CO ratios (<0.5) and H-2 oxidation (undesirable for MTS reaction) at high O-2/CO ratios (>0.5). Furthermore, molar ratios of steam to CO at ranges of 2-4 were compared in OMTS reaction. According to the obtained results, the magnitudes of O-2/CO ratio and S/C ratio which were 0.5 and 4, respectively, were selected as the best values for OMTS reaction. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.