We have studied the reaction between pz-CO2-Re(CO)(3)(bpy) and perchloric acid in acetonitrile by following the UV-vis and IR spectral changes in the reaction mixture. A fast equilibrium was found to be established between solvated protons, pz-CO2-Re(CO)3(bpy), and the protonated intermediate [pz-C(OH)O-Re(C0)(3)(bpy)](+) which finally yields pz-COOH and Re(CO)(3)(bpy)(CH3CN)(+) as reaction products. This intermediate has been characterized by UV-vis and IR spectroscopies and by DFT calculations. The fully optimized DFT/CPCM structures for pz-CO2-Re(CO)(3)(bpy) and [pz-C(OH)O-Re(CO)(3)(bpy)](+) were compared with the X-ray structure of pz-CO2-Re(CO)(3)(bpy). The structural parameters associated with the carboxyl group in the protonated intermediate are between those of pz-CO2-Re(CO)(3)(bpy) and pz-COOH. Multivariate curve resolution methods were employed to obtain the spectrum of the protonated intermediate and the concentration profiles from the full matrix of time resolved UV-vis spectra. The proposed mechanism was numerically simulated by using Runge-Kutta methods. Model parameters were estimated by nonlinear regression fitting of the concentration profiles, yielding values of log(K) = 4.9 +/- 03 and k = 0.16 +/- 0.03 min(-1) for the formation equilibrium constant and the decay rate constant of the protonated intermediate, respectively.