The structure, age and facies distribution of the sedimentary substrate of the Colli Albani volcano has been investigated by analysing sedimentary xenoliths enclosed in pyroclastic and phreatomagmatic deposits from different eruptive centres of the volcano, for comparison with the stratigraphies of deep wells drilled in the area and the known surface geology of the areas surrounding the volcano. Thin-section analysis has been carried out on the same xenolith samples collected and discussed by Funiciello and Parotto (1978. Il substrato sedimentario nell’area dei Colli Albani: considerazioni geodinamiche e paleogeografiche sul margine tirrenico dell’Appennino Centrale. Geologica Romana, 17, 233–287.) and Amato and Valensise (1986. Il basamento sedi- mentario dei Colli Albani: risultato di uno studio degli ejecta dei crateri idromagmatici di Albano e Nemi. Memorie Società Geologica Italiana, 35, 761–767.), largely from the maars deposits of the most recent Via dei Laghi period of activity of the volcano. The pre-orogen marine stratigraphic sequences range from Late Triassic?–Lower Lias to Middle Miocene, with carbonate platform-to-basin facies. These sequences are very similar to those cropping out in the Sabina area to the north and NE of the volcano. Upper Cretaceous carbonate platform units, similar to those cropping out in the Lepini Mts, to the east of the volcano, have been recognized only in the xenoliths from the Nemi maar. Similar to that observed in the outcropping geology of the Sabina region, no evidence of siliciclastic xenoliths related to the Upper Miocene foredeep units have been found. The Upper Miocene?–Lower Pliocene to Upper Pliocene post-orogen marine sedimentary cycle has been recognized in almost all the eruptive centres, and has similarities with the sedimentary units present in the Rome area. The absence of marine Quaternary sediments among the xenolith types suggests that the Colli Albani area was continental before the onset of volcanism.