SrBi2Nb2O9 (SBN) powders and c-oriented thin films have been prepared by sol-gel process. We studied thermal behaviour of this Aurivillius compound by means of X-ray diffraction (XRD) line profile analysis. In the case of polycrystalline samples it is shown that a variation in the number of perovskite layers broadens the XRD lines similarly to stacking faults. Both 'size' and strain effects contribute to the observed width. The diffraction lines exhibited slowly decaying tails in accordance with the presence of highly localised strain fields generating almost pure 'size' broadening. For longer heat treatment the coherently diffracting domain size gradually increases and microstrains are lowered. This may be attributed to the disappearance of faults, i.e. the number of perovskite layers tends to an equilibrium value. On epitaxial films, experiments were performed under fixed incidence and in omega -20 scan mode. In the former situation we observed a displacement of the (001) lines when the incidence angle was slightly changed. In omega -20 scan mode the integral breadth reached values around 1 degrees. When heat treatment time was increased, the width and Lorentzian content of the (001) lines decrease attesting that stacking fault density was lowered.