Non-equilibrium plasma technology provides an unconventional but promising solution for the cleaning of tar contaminated bio-syngas in biomass gasification. This work is focused on the reforming of mixed naphthalene (C10H8) and toluene (C7H8) as typical single ring and double ring tar model components using a gliding arc discharge (GAD) reactor. The influence of naphthalene content, steam/carbon molar ratio and discharge power on the destruction of C10H8 and C7H8 was evaluated to understand their effects on the tar conversion, gas yield, and the energy consumption. Adding H2O to the plasma process forms OH radicals, creating additional reaction routes for the step-wised oxidation of naphthalene, toluene and their fragments towards the production of CO, CO2, and water. The highest conversion of naphthalene and toluene (similar to 80% overall) was obtained when the steam/carbon molar ratio changed between 1.0 and 1.5, which was dependent on the balance between two opposite effects due to the presence of steam: positive effect of OH radicals and negative effect of electron attachment on water molecules. The highest energy efficiency (53.6 g/kWh) was obtained for the conversion of mixed tar compounds, which is by far the highest in previously reported works. CO, H-2 and C2H2 were the major gases with the highest CO yield of 38.6% and H-2 yield of 39.1%. Emission spectroscopic diagnostics was employed to understand the contribution of chemically reactive species to the GAD reaction. Possible reaction pathways in the plasma reforming of mixed naphthalene and toluene were proposed based on the plasma spectroscopic diagnostics coupled with a comprehensive analysis of major gas products and condensable by-products.