This paper is an attempt to contribute to the discussion started in the 1980s and continuing until today, on the origin of orange-brown patinas observed on marble surfaces. SEM and XRD were used in the study of the orange-brown patina growing at the east stoa of Propylaea, at the Acropolis of Athens. The patina consists of an extremely compact, homogenous, 500-microm-thick layer, mainly of hydroxyapatite. The outer part (50 microm) of the layer presents an orange colour attributed to iron in clay particles, which were deposited on the surface of the patina and have gradually been diffused in the bulk. The interface between the phosphatic layer and the marble is very uneven and full of cavities colonised by microorganisms. The phosphatic layer penetrates into the grain boundaries and fills the empty space between the grains. The presence of biological attack at the interface causes progressive erosion and detachment of both the patina and the marble. The minimum presence of oxalates, the absence of microorganisms or remnants of them in the phosphatic layer, the homogeneity and compactness of the patina, give evidence against the theory of biogenic origin. It is suggested that the layer originates from an artificial coating deliberately applied onto the marble surface for protective or aesthetic reasons, when the marble had already been weathered. Possible manmade pastes are suggested.