A dielectric barrier discharge (DBD) has been successfully applied to studying, both theoretically and experimentally, phenol degradation in waste water aqueous solutions. A coaxial reactor was selected where the liquid waste constitutes a part of the internal electrode itself, the liquid solution flowing up inside the hollow internal electrode impelled by a submersible pump. Thus, the solution falls by gravity on the external surface of the internal electrode. The DBD gas flows in parallel to the surface of the liquid. The cold plasma was generated from Ar-O2 mixture and O2 pure with the inclusion of moisture from the same solution. Two power supplies were compared delivering potentials up to 23 kV at 1.5 kHz, and up to 12 kV at 15.6 kHz respectively. The initial concentration of phenol was around 5 × 10−3 mol/L and efficiencies up to 99% were obtained after 1 h of treatment. Finally, a simplified kinetics model was developed where the temporal evolution of the compounds generated in the phenol degradation process was analyzed. Hydroquinone, catechol and resorcinol were obtained as byproducts and H2O, CO2 and some light carboxylic acids as final products.