Two atmospheric-pressure plasma sources were studied. One was a helium plasma generated by a RF discharge, and the other was an air plasma generated by a dielectric barrier discharge (DBD). The two plasma sources were characterized on electron density, emission spectrum, and ozone density. The modification of poly-ethyleneterephthalate (PET) surfaces by the two plasmas was investigated. PET strips were exposed to the plasma at the exit of the plasma source. Water contact angles were measured for surfaces modified with different processing parameters. The change in contact angles was monitored as a function of time. Modification mechanisms were also investigated.