The photografting kinetic of a hindered amine light stabilizer (HALS) was studied in low density polyethylene (LDPE) films under natural weathering conditions. The HALS (PBH-3) was added to the polymer at a concentration of 0.3% (w/w). The photografting kinetic of polymer bound HALS (PBH-3) was determined by direct spectroscopic measurements through the absorption band area of the stabilizer centered at 308 nm in the UV spectra and 1605 cm(-1) in Fourier transform IR (FTIR) spectra, which correspond to benzylidene malonate ester and benzylidene malonic groups, respectively. In parallel, measurements were carried out on the free PBH-3 content after chloroform extraction of the photostabilizer from the polymer matrix by means of UV and gas chromatography methods. The results showed that in natural weathering the grafting of PBH-3 occurred in the LDPE film after very short exposure times of 127 and 168 h as determined by UV and FTIR, respectively. Moreover, the curves describing both the photografting and the free HALS kinetics exhibited similar profiles. The mechanism of photografting implies a rapid photoreaction between the methylenic double bond of the stabilizer and the macroalkyl radicals of the polymer, resulting in formation of polymer-bonded aminyl derivatives of the stabilizer.