Raman spectroscopy is an analytical technique, which is gaining attention as a molecular technique for the investigation of objects of art. Especially the non-destructive properties of the method make this application suitable for the in situ analysis of artefacts. However, although using mobile, fibre optics Raman instrumentation for this type of research seems to be straightforward, some practical obstacles may hamper the investigation. In this paper, pitfalls and solutions are described when applying a dedicated spectrometer to the analysis of mediaeval wall paintings. It is shown how some practical problems may be overcome, and the results of the analysis are presented. Although the mediaeval wall paintings from the chapel of the castle of Ponthoz are well-preserved, still some interesting degradation phenomena could be observed: the identification of a black degradation product, likely to be meta-cinnabar, a degradation product of the red pigment vermilion (HgS); the formation of gypsum (CaSO4.2H2O) as a weathering product of calcium carbonate (CaCO3); the observation of copper(II)hydroxychlorides.