The electrochemical anodic dissolution of sputter-deposited, aluminum-doped zinc oxide (ZnO:Al) was investigated. The dissolution proceeds mainly via a local oxygen evolution reaction and subsequent dissolution of the zinc oxide lattice. The present work focuses on the influence of the anion species in the used electrolyte. Several electrolytes (KCl, K2SO4, KNO3, KClO4) were applied. The anion species turned out to be a major factor that influenced the kinetics of the etching process. In K2SO4 solutions a strong limitation of the etching process to the close vicinity of the grain boundaries was observed. However, anodic dissolution in KCl, KNO3, and KClO4 showed an unexpected etching behavior, which lead to the formation of cavern-like structures within the ZnO:Al thin films. This was attributed to different local pH values at the ZnO:Al/electrolyte interface.