We report on the observation of hot water vapor ( steam ) in the inner AU of a young massive star located in the star-forming region IRAS 08576-4334 . The water lines are detected in a medium resolution ( R \sim 10,000 ) K -band spectrum taken by the infrared spectrometer ISAAC mounted on the VLT-ANTU . The water vapor is at a mean temperature of 1565 \pm 510 K , cooler than the hot CO found in the same object , which is at \simeq 1660 K and the column density is N ( H _ { 2 } O ) = ( 2.5 \pm 0.4 ) \times 10 ^ { 18 } cm ^ { -2 } . The profile of both H _ { 2 } O and CO lines is best reproduced by the emission from a Keplerian disk . To interpret the data , we also investigate the formation of molecules and especially CO and water vapor in the inner hot and dense part of disks around young high mass stars using a pseudo time-dependent gas-phase chemical model . Molecules are rapidly photodissociated but this destruction is compensated by an efficient formation due to fast neutral-neutral reactions . The ability of CO molecules to self-shield significantly enhances its abundance . Water molecules are sufficiently abundant to be detectable . The observed H _ { 2 } O/CO ratio is reproduced by gas at 1600 K and an enhanced UV field over gas density ratio I _ { \mathrm { UV } } / n _ { \mathrm { H } } =10 ^ { -4 } –10 ^ { -6 } . The simulations support the presence of CO and H _ { 2 } O molecules in the inner disks around young massive stars despite the strong UV radiation and show that the OH radical plays an essential role in hot gas chemistry .