This paper analyses the effects of time storage on the surface properties of plasma-treated poly(ethylene terephthalate) (PET) plates. For this purpose, optical profilometry-surface topography and surface composition (XPS technique) of the samples were studied before and after plasma treatment. Contact angles were measured using water, ethylen glycol and diiodomethane as liquid probes. Tadmor’s theory was used to calculate the equilibrium contact angles. Apparent surface free energy (SFE) was determined using two approaches, the hysteresis CAH approach and the acid-base LWAB. The results showed a significant increase in wettability after plasma treatment, especially in the case of argon-plasma activation. Surface roughness and the polar component of the SFE increased after activation, while the nonpolar component of the SFE did not change. It can be concluded that PET surface roughness is increased by air-plasma activation due to the polar groups originated by the treatment. The surface is not stable during storage time and becomes more hydrophobic because of surface layer reorganization. In the case of argon-plasma activation, the effects are stable over time, which was discussed using two different apparent surface free energy calculation approaches.