The thermal stability and dewetting effects of crystalline organic thin films on inorganic substrates have been investigated for a model system for organic epitaxy, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Ag(111). The thin films deposited under a variety of growth conditions have been annealed stepwise and studied by in situ x-ray diffraction and noncontact atomic force microscopy. It has been found that comparatively smooth films deposited at temperatures T(g)less than or similar to350 K are metastable, while films deposited at T(g)greater than or similar to350 K are stable against annealing on a time scale of several hours. The thermal expansion of thin PTCDA films and bulk samples has been determined. (C) 2003 American Institute of Physics.