Saint-Jean-Baptiste de la Cité in Périgueux (Dordogne, France) is a historical monument that has been classified for its high quality Renaissance style. Recent archaeological works (2008) had revealed seven states of construction and transformation since the Gallo-Roman Antiquity until the present[1]. The oldest phase of construction is dated to the end of the 3rd c. AD according to the coins that were found in layers in connection with the corresponding masonries. The reconstruction of the choir in the Renaissance style is precisely dated 1521 AD by an inscription. This phase and the Gallo-Roman one are the only periods that are precisely known. Between them, two successive periods of construction were defined the dates of which were uncertain until now: i) a polygonal apse building associated with a cocciopesto floor (lime mortar with fragments of bricks), that is identified as a possible element of the paleo Christian episcopal complex of Périgueux. ii) the square chevet of a medieval episcopal chapel. Our study aims at determining the absolute chronology of the building sequence and particularly these two imprecisely dated phases. For that purpose, single grains OSL dating was used[2]. Numerous samples of mortar and sediments were sampled. The SG-OSL results on mortars and sediment taken from the well dated structures are in agreement with the expected chronology. The SG-OSL results also indicate that the polygonal apse was built around the 4th – 5th c. AD. and made probably part of a larger primitive Christian complex. An early medieval structure built within the 7th-8th c. AD has also been identified. The plasters analyzed from the elevated parts interpreted as the remains of the medieval choir are related to the Romanesque period, between 10th-12th c. AD. This wall could have been neither probed, nor sampled. At this point of the study we are not able to clarify if the wall made part of a Romanesque reconstruction, or if it is related to a repointing of the internal early medieval building. Finally, the northern wall of the nave, initially assumed to be medieval, was found to be in fact a Gallo-Roman wall, integrated in the successive constructions. Our research on the chapel of Saint-Jean-Baptiste is one of the first case studies that clearly illustrates the potential and relevance of the SGOSL mortar dating for building archaeology, in particular for deepening knowledge on many poorly known early medieval monuments. [1]GAILLARD H., 2015, Un nouvel élément dans la topographie chrétienne de Périgueux : la chapelle SaintJean- Baptiste à la Cité, in CARTRON I., HENRION F., SCUILLER C. (dir.) : Les sarcophages de l'Antiquité tardive et du haut Moyen Âge : fabrication, utilisation, diffusion, Actes des XXXe Journées internationales d'archéologie mérovingienne, Bordeaux 2009. [2]URBANOVÁ P. & GUIBERT P., 2017. A methodological study on single grain OSL dating of mortars : comparison of five reference archaeological sites, « Geochronometria », 44, pp. 77-97.