The once colourful mural paintings at the Beni Hasan tombs (ca. 2100 B.C.), an important Egyptian archaeological site, are covered with an obscuring grey deposit. To study this surface degradation, x-ray photoelectron spectra of fragments from the painted walls have been collected. Three types of surfaces were identified. A preliminary mechanism for opaque layer formation is proposed. It suggests that the principal physicochemical processes are adsorption of water in the presence of carbon dioxide, followed by dissolution of the substrate walls upon which the murals were painted. This mechanism is consistent with the finding that mural deterioration is recent; it also indicates that deterioration is progressive, continuous, and cumulative. Relatively recent efforts in removing obtrusive surface layers with dilute hydrochloric acid (HCl) were found to be performed carefully; there was no evidence of excess chlorides on cleaned surfaces. However, this finding does not provide a guarantee that murals were undamaged by hydrochloric acid (HCl) below the surface. It does suggest that progressive damage due to the formation of hygroscopic calcium chloride (CaCl₂) on the surface has been avoided. -- AATA