Thallium(I) hydroxide (TlOH) is an inorganic compound with the chemical formula TlOH. It is a strong base, typically obtained as a yellow solid or in solution. It is highly toxic, hygroscopic, and readily absorbs carbon dioxide from the air to form thallium(I) carbonate. Its primary application is in chemical synthesis and materials science. In organic synthesis, it serves as a source of the thallium(I) ion (Tl⁺) for specific reactions, such as the preparation of cyclopropanes via the Simmons-Smith reaction when used with diiodomethane. In materials research, thallium compounds, including TlOH, are investigated as precursors for the synthesis of thallium-based high-temperature superconductors (e.g., thallium barium calcium copper oxide, TBCCO) and other advanced electronic materials like thallium halide perovskites for photovoltaics and radiation detection. Due to the extreme toxicity of all thallium compounds, handling requires stringent safety protocols, including the use of appropriate personal protective equipment (PPE) and engineering controls in a well-ventilated fume hood. Its use is strictly limited to specialized research and industrial settings.