The stability of thin diblock copolymer films with respect to the annealing above the microphase separation temperature is investigated on long time scales and at different film thicknesses. Poly(styrene-block-p-methylstyrene) diblock copolymer films on top of silicon substrates are examined with scanning force microscopy and off-specular X-ray scattering. At film thickness below the lamellar spacing of the bulk material a dewetting is observed. The kinetics of the film destabilization are explainable within a spinodal dewetting model. The observed dewetting structures belong to a wet dewetting. In the case of film thicknesses which enable the buildup of a few lamellae no sign of instability was detected.