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Atomic layer deposition (ALD) is a technologically important method to grow thin films with high conformality and excellent thickness control from vapor phase precursors. The development of new thermal ALD processes can be limited by precursor reactivity and stability: reaction temperature and precursor design are among the few variables available to achieve higher reactivity, unlike in solution synthesis where the use of solvent and/or a catalyst can promote a desired reaction. To bridge this synthesis gap between vapor and solution, we demonstrate the use of an ultrathin coating layer of a vapor-phase compatible solvent—an ionic liquid (IL)— onto our growth substrate to perform ALD of SnO. Successful SnO deposition is achieved using tin acetylacetonate and water, a process that otherwise would require a stronger counter-reactant such as ozone. The layer of IL allows a solvent-mediated reaction mechanism to take place on the growth substrate. We report a growth per cycle of 0.67 Å/cycle at a deposition temperature of 100 °C in an IL comprised of 1-ethyl-3-methylimidazolium hydrogen sulfate. Characterization of the ALD films confirms the SnO film composition, and 1H and 13C NMR are used to probe the solvent-mediated ALD reaction, suggesting a solvent-mediated addition-elimination type mechanism forming acetone and acetate. Density functional theory calculations show that the ionic liquid solvent is beneficial to the proposed solvent-mediated mechanism by lowering the C-C bond cleavage energetics of acetylacetonate compared to the vapor phase. A general class of ligand-modification reactions for thermal ALD is thus introduced in this work.