Structure of ruthenium catalysts deposited on various supports of different surface area and acid-base properties: Fe2O3, La2O3, MgO and C-act, active in the water-gas shift reaction (WGSR), was studied by XRD, TEM, electron diffraction and low temperature nitrogen adsorption. It has been found that decreasing activity of the catalysts in the WGSR: Ru/La2O3>Ru/Fe2O3 (D)>Ru/Fe2O3 (B)>Ru/ MgO>Ru/C-act corresponds to increasing mean size of Ru particles in the catalysts determined by TEM: 1.11, 1.93, 2.80, 3.53 and 3.43 nm, respectively. No direct relation was established between the porous structure of the supports and the catalytic activity of the ruthenium catalysts supported on them.