A kraft lignin derivative (KLD) obtained by reaction with p-aminobenzoic acid/phthalic anhydride, was blended with poly(vinyl alcohol) (PVA) by solution casting from DMSO. PVA and PVA/KLD films were exposed to ultraviolet radiation (Hg lamp, 96 h) and analyzed by thermogravimetry (TG) in inert and oxidative atmosphere. Typical multi-step decomposition profiles were obtained. The apparent activation energy (E-a) of the thermal degradation of the samples was computed by the Vyazovkin method. The KLD degradation presented only small intervals of decomposition degree with constant E-a values. PVA and blends showed intervals of up to 50% in decomposition degree with nearly constant E-a, and smaller intervals in which E-a varies drastically. The influences of samples irradiation and of surrounding gas in TG analysis on E-a are also shown. (C) 2005 Elsevier B.V. All rights reserved.