The electro-oxidation of p-Hydroxymandelic acid has been studied in acid (0.5 M H2SO4) and alkaline solutions (0.1 M and 2.7 M NaOH) using voltammetric and in situ Fourier trans-form infrared (FTIR) spectroscopic techniques. By using chemical analysis, voltammetric data, and FTIR spectra obtained during the oxidation of p-hydroxymandelic acid, p-hydroxybenzaldehyde, and CO2 have been identified as the main reaction products in both media. In sulfuric acid, the electro-oxidation suffers a deactivation with continued sweeping. However, in alkaline solutions the current density is stable with sweeping and increases with NaOH concentration.