Polyurethane (PU) and polystyrene (PS) films were functionalized by ultraviolet (UV) or selective synchrotron radiations (SR) in the presence of reactive gases. The UV-PU results were compared with low-pressure plasma treatments of the same films. Oxygen or acrylic acid vapours (AA) were used as reactive gases. X-ray photoelectron spectroscopy measurements of UV modified films in the presence of oxygen or AA matched the RF-plasma treatments results. It is shown that COO and C=O functional groups were incorporated at the polymer surface efficiently with both methodologies. In addition, near-edge X-ray absorption fine structure showed that a thin film of poly(acrylic acid) is formed over the PU and PS films during the UV irradiation in the presence of AA vapours. These results resemble previous AA low-power plasma treatments. PU and PS films were also selectively functionalized by SR using oxygen as reactive gas. Surface concentrations of COO and C=O functional groups were enhanced by C-1s -> sigma(C-C)* excitation after irradiation and oxygen introduction. This efficient surface functionalization was clearly observed in PS films which do not have CO and COO groups in their molecular structure. Excitations involving transitions to pi* orbital (pi(C=C)*, pi(C=O)*) led to much lower functionalization efficiency. The SR results can be explained by taking into account previous photon stimulated ion desorption studies of polymers. SR results may open new ways to functionalize polymer surfaces selectively and efficiently. (C) Koninklijke Brill NV, Leiden, 2010