The influence of temperature treatment on three types of conjugated polymer thin films, named poly(3-hexylthiophene) (P3HT), polyarylamine (PAA) and octylfluorene-bithiophene copolymer (F8T2) is studied. A detailed knowledge of the film morphology and crystalline structure is important since the performance of organic thin film transistors is extremely sensitive to small changes of morphology and structure. Samples are prepared via a spin-casting process on thermal oxidized silicon wafers. The influence of heat treatment in the range from ambient temperatures to 600 K is studied with specular X-ray reflectivity, X-ray diffraction and differential scanning calorimetry. The morphological parameters like layer thicknesses, electron densities and roughnesses of the interface and of the surface are calculated from the XRR measurements. The maximum change of layer thickness due to heat treatment is 15% of the initial layer thicknesses. The maximum variation of the mean electron densities are about 20% and the rms surface roughness vary from 2 angstrom up to 20 angstrom as a result of annealing. Interface roughness show variation of about 1 angstrom. Strong variations of the morphological parameters next to phase transition temperatures are observed as well. (c) 2006 Elsevier B.V. All rights reserved.