This paper describes the composition, decay patterns and degradation mechanisms of the building stones and mortars of the Cathedral of Lleida (Catalonia, Spain). Various areas present considerable damage. Four forms of decay are apparent: grain disintegration, joint mortar loss, flaking and efflorescence. The type of stone, its composition and the orientation of the building determine the appearance of the decay forms. The paper analyses these altered mineral forms in the monument, and discusses their possible origin at the level of mineral reactions and stability.