We present a comprehensive study of the temperature dependence of ultraviolet-visible ellipsometric spectra of thin films of poly (methyl-phenylsilane). We have found the threshold of irreversible changes of its optical response at 373 K. We have found the average temperature shift of the lowest excitonic band of - (8.5 +/- 0.8) X 10(- 4) eV/K in the reversible regime. The effect of annealing below and above 373 K is studied with a low- and high-level of the exposure to ultraviolet light. (C) 2004 Elsevier B.V. All rights reserved.