Perfluoroalkanoyl peroxides such as (C3F7COO)2 have been found to react with polyester film only on the surface. The modified polyester surfaces were characterized by contact angle measurements using a two-liquid phase system and by ESCA spec troscopy. These provided surface free energy values and surface atom content values as a function of reaction time. Perfluoroalkyl groups substituted on the aromatic rings of the polyester surface influenced both the dispersive component γsd of surface free energy and the polar one γSp. The surface free energies decreased with the reaction time and reached a plateau at about 1 hour. There was a large decrease in γSd. Based on the ESCA characterization, the area ratio of the F1 s peak to the O 1 s peak was linearly correlated not only to γ Sd but also to γSp.