The gas-solid interface of ceramics is a key aspect, since it is where reactions occur and sensing signals are generated. Therefore the chemistry of as-grown mullite-type Bi2Ga4O9 (110) single-crystals surfaces and its change due to thermal treatments (500 degrees-800 degrees C) under ultra high vacuum conditions and irradiation by low energy ions (Ar+) have been investigated by in situ X-ray photoelectron spectroscopy. Ion bombardment causes preferential sputtering of O atoms bonded to Bi, and partial reduction of Bi3+ to metallic Bi0, whereas Ga3+ is not affected. Upon annealing under vacuum conditions, simultaneous O and Bi interdiffusion from the bulk of the crystals and segregation onto the surfaces on the one hand, and sublimation of metallic Bi present on the outermost surface layers on the other hand have been observed. Because of these dynamical processes the chemical composition of the surface, above 600 degrees C recovers to the nominal (110) surface stoichiometry and to the Bi3+ and Ga3+ oxidation states of Bi2Ga4O9. The same effects were found by annealing the single crystals in air at 700 degrees C. Changes of the morphology of the mullite-type Bi2Ga4O9 (110) single-crystals surfaces upon ion irradiation and thermal treatments have been monitored by atomic force microscopy. Ion irradiation produces a significant sputtering-induced increase of the surface roughness. The smooth pillow-like structure of the irradiated surfaces after annealing at 700 degrees and 800 degrees C may indicate the partial formation of a glassy or pseudo-amorphous phase.