The present disclosure provides a non-isothermal wet atomic layer etch (ALE) process for etching polycrystalline materials, such as metals, metal oxides and silicon-based materials, formed on a substrate. More specifically, the present disclosure provides various embodiments of methods that utilize thermal cycling in a wet ALE process to independently optimize the reaction temperatures utilized within individual processing steps of the wet ALE process. Like conventional wet ALE processes, the wet ALE process described herein is a cyclic process that includes multiple cycles of surface modification and dissolution steps. Unlike conventional wet ALE processes, however, the wet ALE process described herein is a non-isothermal process that performs the surface modification and dissolution steps at different temperatures. This allows independent optimization of the surface modification and dissolution reactions.