Polystyrene thin films undergo treatment by cold plasma produced from DC point-to-plane low- pressure (4 mbar) discharge in O2 and Ar. The surface tension highly increases (more than 70% for O2 plasma and 60% for Ar plasma) by applying as weak as 200 μA discharge mean current. The treatment time required for obtain the maximum wettability is double in the case of Ar (~60 s). Important increase in surface polar component takes place while the dispersive component decreases, regardless of the gas used. Oxygen-functionalities (10–12%) graft the first 5 nm of surface and the surface polarity is decupled, in both gases. The main part (>99.8% for O2 and >97% for Ar) of the treatment time corresponds to a relaxation phase of the discharge (post-discharge time) and, thus, the role of long lifetime excited particles (>3 eV) in the activation of the polymer surface is emphasized. An ionization front having velocity ~104–106 m/s (higher in O2 than in Ar) crosses the gap starting from the anode and propagating towards the cathode. The arrival of this wave at the cathode region boosts the cathode secondary processes. The secondary electrons amplify the discharge current and trigger off the initiation of a new ionization front, stronger than the first one, which establishes the glow discharge regime. These fronts (weaker in Ar discharge) are suggested as the main mechanisms for the cold plasma production and active particles generation for the polymer treatment.