We show HO is spontaneously produced from pure water by atomizing bulk water into microdroplets (1 μm to 20 µm in diameter). Production of HO, as assayed by HO-sensitve fluorescence dye peroxyfluor-1, increased with decreasing microdroplet size. Cleavage of 4-carboxyphenylboronic acid and conversion of phenylboronic acid to phenols in microdroplets further confirmed the generation of HO The generated HO concentration was ∼30 µM (∼1 part per million) as determined by titration with potassium titanium oxalate. Changing the spray gas to O or bubbling O decreased the yield of HO in microdroplets, indicating that pure water microdroplets directly generate HO without help from O either in air surrounding the droplet or dissolved in water. We consider various possible mechanisms for HO formation and report a number of different experiments exploring this issue. We suggest that hydroxyl radical (OH) recombination is the most likely source, in which OH is generated by loss of an electron from OH at or near the surface of the water microdroplet. This catalyst-free and voltage-free HO production method provides innovative opportunities for green production of hydrogen peroxide.