Controlled atmosphere electron microscopy (CAEM) has been used to observe and compare directly the behavior of palladium particles supported on gamma-alumina, silica and zirconia, when heated in the presence of oxygen, Palladium is converted to PdO on all three supports at temperatures in excess of 325 degrees C and undergoes a wetting and spreading action to form thin flat crystallites. At temperatures above 750 degrees C a significant fraction of the PdO disproportionates to form metallic Pd, which subsequently undergoes a rapid interaction with both alumina and silica at 800 degrees C to form compounds that probably consist of mixtures of the respective oxides; however, the precise composition of these species cannot be determined, It is suggested that this phenomenon is responsible for deactivation of these catalyst systems at high temperatures. This type of interaction was not observed when zirconia-supported palladium specimens were treated under the same conditions, In this case, the support appeared to stabilize the formation of PdO up to 900 degrees C. It is significant that on all supports a fraction of the palladium remains in the form of PdO up to the highest temperatures even though bulk thermodynamics predict that it should all be in the metallic state.