Thermal effects on optical properties and non-linear optical absorption have been studied in hydrogenated amorphous silicon films using dual beam thermal lens technique. A change of the thermal lens signal, from positive to negative, with increase of the excitation power at some wavelengths, has been observed. The thermal conductivity and thermal diffusivity were K = (2.6 +/- 0.1) x 10(-3) W/cm K and D = (1.9 +/- 0.1) x 10(-3) cm(2)/s, respectively. Moreover, we observed a decay in the transmission signal, which was fit with two exponential functions, related to thermal effects, and competition between the creation of photo induced defects and their thermal annealing. (C) 2004 Elsevier B.V. All rights reserved.