This paper presents the study of the nature and conservation state of different stuccos and Roman concretes from the Roman city of Baelo Claudia (Cádiz). The chemical, mineralogical and microstructural compositions of the said materials have been established. Different causes of their decay have been found, among which we should emphasize the loss of the binding material (lime), basically due to the processes of partial solubilization; the lack of bond between the stuccos and the support stone, due to the different characteristics and properties of these two materials; and finally, the biological-type attack with the proliferation and colonization of different organisms in the said materials.