We investigated the catalytic activity of Co/K/alpha-Al2O3 with various potassium (K) loadings for ethanol steam reforming. The promotion effect of K loading and the role of K in Co/K/alpha-Al2O3 were investigated using TEM-EDX, H-2-TPR, XAFS, XPS, and IR measurements. Results show that loading of 1-2 wt% K on Co/alpha-Al2O3 was effective for improving catalytic activity, selectivity to hydrogen and suppressing by-product (CH4, C2H4, and coke) formations. Measurements by H-2-TPR and XAFS showed that the reducibility of the supported Co species decreased by K loading. TEM measurements revealed that the Co-0-CoO core-shell structure was formed over Co/K/alpha-Al2O3 during ethanol steam reforming at 823 K, suggesting that the oxidized Co species (CoO) is a highly active species in ethanol steam reforming. In situ IR measurements revealed that the adsorbed ethanol forms stable acetate species by K loading, which improves hydrogen selectivity. (C) 2015 Elsevier B.V. All rights reserved.