The oxidation of the metallic components in the semiconductive Ba₈Ga₁₆Sn₃₀ type-VIII clathrate, a good candidate for thermoelectric materials utilized at temperatures around 250°C, has been observed during crystal growth in vacuum. In this study, Ba₈Ga₁₆Sn₃₀ samples were sealed in vacuum and kept at 400°C for various times. The morphology and composition of oxide layers were analyzed by means of SEM and XPS respectively and the influences of the oxidation on the thermoelectric properties including the Seebeck coefficient and power factor of Ba₈Ga₁₆Sn₃₀ were investigated. The compositional depth profile in the oxide layer shows that barium concentration decreases but gallium and tin concentrations increase with depth. The chemical states of the components were also analyzed. It was also found that pure tin clusters form on the surface beneath the oxide layer.