The influence of different sample pretreatment methods in the period between switching off the potential control and the actual X-ray scan on the results of ex situ X-ray powder diffraction analysis of thin corrosion films of Pb anodized in H2SO4 has been studied. Washing the sample with a water (or acetone or ethanol) spurt and drying it rapidly with compressed air or nitrogen immediately after switching off the potential and removing the electrode from the solution, gave the most reliable and reproducible results. The washing was essential to prevent the discharge of PbO2 to PbSO4 on the electrode surface. The moisture remaining in the corrosion layer was found to render a self-discharge reaction inside the film possible, thus changing the original composition of the layer. It is concluded that more discussion is needed in the literature on sample pretreatment before ex situ analysis.