Absolute dating of construction materials represents one of indispensable means for building history. Recent researches [1] showed that in the Middle-Ages a re-use of bricks originating from older structures was a common phenomenon. Therefore, dating of bricks is not always representative for the erection of the masonry, the moment to be dated. That is why we focus on mortar. Current attempts in mortar dating concentrate on the radiocarbon method which has, however, its limitations [2,3]. Thus, dating of mortars by optically stimulated luminescence (OSL) may become an interesting alternative. The basic premise in such analysis is that quartz in the sand used for making mortar is optically zeroed during the preparation process. The moment to be dated is the last exposure of mortar to light, before being embedded within the masonry and hidden from light. The first goal was to evaluate the reliability of the method by dating the mortars from well-dated Roman and medieval reference structures and to understand all the difficulties encountered [4]. Optical bleaching of certain grains can be partial due to the short exposure to light, which indicates the necessity to work by the single grain technique in order to find and select the well-zeroed grains. The studied material is also often very coarse, which causes the heterogeneity of irradiation at the grain scale, and thus the dispersion of equivalent doses. We tried to understand this phenomenon by using EDX-SEM cartography, autoradiography beta and ICP-MS. As a result of this 3-year project, we are able to present today the convenient methodology for dating mortars by OSL obtaining a good agreement between OSL and reference ages for many samples [5]. The promising potential of the method will be demonstrated on undated mortars sampled during the archaeological research in the crypt of the Saint Seurin basilica, Bordeaux. [1] P. Guibert, I. Bailiff, M. Baylé, S. Blain, A. Bouvier, S. Büttner, A. Chauvin, Ph. Dufresne, A. Gueli, Ph. Lanos, M. Martini, D. Prigent, C. Sapin, E. Sibilia, G. Stella, O. Troja, The use of dating methods for the study of building materials and constructions: state of the art and current challenges, Proceedings of the 4th International Congress on Construction History, Paris 3-7 July 2012, 469-480, 2012. [2] G. Pesce, R. J. Ball, Dating of Old Lime Based Mixtures with the Pure Lime Lumps Technique, In: Nawrocka D. M. (ed.) Radiometric Dating, InTech, 21-39, 2012. [3] F. Marzaioli, C. Lubritto, S. Nonni, I. Passariello, M. Capano, F. Terrasi, Mortar Radiocarbon Dating: Preliminary Accuracy Evaluation of a Novel Methodology, Analytical Chemistry 83(6), 2038-2045, 2011. [4] P. Urbanova, D. Hourcade, C. Ney, P. Guibert, Sources of uncertainties in OSL dating of archaeological mortars: the case study of the Roman amphitheatre Palais-Gallien in Bordeaux, Radiation Measurements, 72, 100-110, 2015 [5] P. Urbanova, P. Guibert, New insights towards the dating of Roman and medieval mortars by Optically Stimulated Luminescence (OSL) : comparison of case studies, Proceedings of the 5th International Congress on Construction History, Chicago, Etats-Unis, 3-7/6/2015.