We have studied the ultraviolet photoreaction of HBr . acetone complexes isolated in argon matrices using Fourier-transform infrared spectroscopy. Photoexcitation of the acetone pi*<--n(0) transition, using either 266 or 309 nm, results in the reaction of hydrogen-bonded HBracetone complexes to produce HBr . propen-2-ol complexes through a novel photoenolization mechanism. Only the hydrogen-bonded HBr . acetone complexes undergo reaction following mid-UV irradiation. No significant loss of isolated reactant molecules occurs using either 266 or 309 nm excitation. Detailed ab initio calculations of HBr . propen-2-ol complex are presented to support these findings. Photoexcitation of similar matrices at 193 nm, which excites the acetone 3s<--n(0) transition and HBr directly, shows extensive reaction of HBr, acetone, and their complexes by a photofragmentation mechanism. We discuss the mechanism of HBr assisted acetone photoenolization in detail and report experimental and calculated band frequencies and structures of HBracetone complexes.