AFM has been used to study the effects of pretreatment gases on Pd/SiO2 supported thin film catalysts during 1,3-butadiene hydrogenation. The Pd/SiO2 catalyst, treated with O2 followed by H-2 at 450-degrees-C, has an initial conversion of 85% and a surface morphology of 60 x 65 nm2 Pd grains and only deactivates slightly. After a second treatment, the reactivity was fully recovered and the surface morphology exhibits a redispersion of the Pd grains. The catalyst with a similar initial reactivity and morphology but only treated in H-2, shows a decrease in activity and coalescence of Pd grains after repeated treatment. XPS studies have shown that the O2 and H-2 treated Pd/SiO2 catalyst has a lower Pd binding energy than the H-2 treated Pd/SiO2. The effect of the substrate thickness and composition is also reported.