The valence-band ultraviolet photoemission spectrum of ${\mathrm{VO}}_{2}$ has been studied above and below the semiconductor-metal phase-transition temperature (\ensuremath{\approxeq}340 K) as a function of exposure to hydrogen. Evidence is found for a partial reduction of the surface, leading to formation of adsorbed OH and an increase in the V 3d-electron density. The adsorbed hydrogen appears to impede the phase transition within the photoemission sampling depth. A model for this effect is suggested, based on an increased V-O \ensuremath{\pi}-bonding interaction.