This article presents the "in situ" and totally non destructive investigation of a wall painting in Santa María de Lemoniz (Biscay, Basque Country, Spain) by Raman microprobe spectroscopy 14 years after its restoration. Although no sample was allowed to be taken, it has been possible to determine the original pigments in the artwork (vermilion, red iron oxide, yellow iron oxide, carbon black, lead white), as well as some degradation products (calcium oxalate dihydrate, anhydrite). For the first time, the mechanism for the transformation of malachite into copper basic sulphates has been ascertained by the integration of Raman data with thermodynamic speciation studies. Moreover, some remarks regarding the unsuitability of the past intervention procedure with regard to the chemical stability of the artwork are made.