Core-level and valence-band photoemission have been used to investigate oxidation at the K/GaAs(100) interface. The results show that potassium deposited on the top of a substrate forms a polarized layer. The K-promoted oxidation mainly involves As. The initial bonds between an alkali metal and a semiconductor have less of a relation with the amount of adsorbed oxygen, but the disruption of K-As bonds restores a large density of As dangling bonds, and the adsorbed oxygen coupling of these dangling bonds leads to the out-of-proportional function of the As oxide.