Supported manganese oxide catalysts were prepared by impregnating manganese salts onto TiO 2, CeO 2, Al 2O 3 and MgAl 2O 4 and precipitating Mn(OH) 2 in situ using ammonium hydroxide. X-ray diffraction and ion scattering spectroscopy showed that the preparation procedure left well-dispersed overlayers on the support surfaces without formation of separate crystalline Mn oxide phases. The catalytic activity for ethylene hydrogenation varied by over three orders of magnitude on all the catalysts. The changes are a function of the support and are apparently related to the reducibility of the surfaces, the hydrolysis of Mn-support bonds and/or structural changes in the MnO x overlayers.