The effects of laser irradiation and annealing of rhodium phthalocyanine Langmuir-Blodgett films deposited on quartz and Au-covered glass substrates were studied with ultraviolet-visible and Fourier transform infrared spectroscopy, X-ray diffraction and atomic force microscopy. We found that laser irradiation and annealing produce very similar results attributed to the heating effect in both cases and produce different results in different atmospheres (air and Helium). Changes observed in, films treated, at 200 degrees C in an inert atmosphere are attributed to a molecular rearrangement of the films associated with the destruction of J aggregates. Films treated in similar conditions in air show, besides this effect, possible oxidation and loss of molecules. A remarkable decrease of toughness was observed when the films are annealed at 100 degrees C in He, attributed to the evaporation of water molecules trapped in the Langmuir-Blodgett films. (c) 2005 Elsevier B.V. All rights reserved.