Atomically flat oxide single crystals, some of which have now become commercially available, are getting more and more important today. It is because they can provide a useful platform not only in high-quality oxide epitaxy and its device applications, but also in fundamental investigations of oxide surface science. In this review, I summarize some of the wet processes that have made a breakthrough in the preparation of such an atomically flat oxide single crystal surface, focusing on the chemistry involved in these wet processes. It includes those of TiO₂ and MgO(001) oxide single crystals, and the benefits we can get from the use of these atomically flat oxide single crystals are also displayed.