The gas phase ultraviolet spectrum and stability of 1,1-dihydroxyacetone (the geminal diol of methylglyoxal) are investigated using spectroscopic and computational methods. Experimental gas phase electronic spectra recorded in the ultraviolet–visible range were used to follow the hydration of methylglyoxal. We show that upon the addition of water, methylglyoxal hydrates to form the geminal diol. The electronic spectra of methylglyoxal and its geminal diol conformers were calculated using TD-DFT and LR-CC methods to facilitate experimental peak assignments. The lowest energy electronic transition of the diol is experimentally measured and compared with the results of the theoretical calculations. We find that upon formation of the diol, the lowest-energy transition of methylglyoxal (centered around ∼430 nm) vanishes, and the remaining transitions, including the methylglyoxal diol absorptions (λmax = 240 nm), fall outside the available solar actinic flux in the troposphere. This likely has significant impac...