The contraction of coordinated aromatic hydrocarbons is a rare reactivity pattern in organometallic chemistry. We describe the conversion of a. bromobenzene coordinated to a Re-I center into a cyclopentadienyl aldehyde. Under mildly alkaline conditions, the expected phenol complex is formed with Re and Tc-99 but under strong basic conditions; ring contraction occurs in close to quantitative yields for Re only. A mechanism for this unprecedented reaction is proposed based on H-1 and H-2 NMR spectra and density functional theory calculations.