The kinetics of the dopant-deriving products formed upon photolysis of the photolabile iodonaphthalene and iodophenanthrene dispersed within PMMA films is examined at 248 and 193 nm. The products include ArH, formed via a thermally activated process, and NaP2, formed via diffusion-limited reaction. Therefore. their formation kinetics affords a direct probe of the temporal evolution of the temperature and of the viscosity of the polymer upon UV irradiation. It is demonstrated that at both wavelengths, kinetics is well accounted by a thermal mechanism. However, the viscosity changes deviate much from conventional measurements. In addition, the results illustrate the importance of the interplay of temperature evolution and of the induced transient polymer melting on product formation. (c) 2005 Elsevier B.V. All rights reserved.