The mural paintings on the wall of the above cited convent composed of Carolingian plasters and frescoes over which Romanesque ones are laid have suffered accelerated decay since they were restored in 1947-1951. A table lists the main events that have affected them since the year 800, but the main deterioration is caused by salt concentration originating in rising damp. The moisture distribution, illustrated by graphs, reflects that of hygroscopic salts and not what could be expected from rising damp without salt concentrations. Analysis determined that the decay was produced by soluble salts, and the crystallization occured as a result of fluctuations of the relative humidity which in turn depends on climatic variations. Water percolation is also a factor. Since the last 1947-1951 restoration, the walls have been partially dried and the church heated, which has caused stronger seasonal variations of the RH, the main cause of the increasing speed of deterioration. There is no simple way to preserve the wall paintings. Four small, corrigible steps to reduce the frequency of the crystallizations have been undertaken and are described. Acknowledgements are appended.