The Coudenberg archaeological site consists of the remains of the former Palace of Brussels, made up of successive building phases ranging from the 12th to the 18th century. After the fire of 1731, the ruins of the Palace were demolished for the purposes of a new urban project: the Royal Square. Although the Palace disappeared from the urban landscape, remains of the basement level are still present beneath the square, and have been made accessible following excavation campaigns carried out over the past twentyfive years. In the case of this study, 17 mortars belonging to the basement of various well-dated building phases were characterized by means of a combination of techniques (optical and scanning electron microscopy, simultaneous thermal analysis as well as an acidic treatment) aiming to identify the mortar composition and to find out the geological source of the raw materials. The binder of the mortars from the 12th to the 17th century is made up of slightly hydraulic lime. The systematic presence of quartz-bearing lime lumps, burnt glauconite grains and ghosts of microfossils/bioclasts allows identifying the raw material used for lime production: the microsparitic sandy limestone rubbles in Brusselian sands of Eocene age (Brusselian stone). The use of Brusselian lime is confirmed by archival documents contemporaneous to the Palace’s construction. The granulometry of the sand used as aggregate is comparable to that of Brusselian sands. The binder/sand ratios determined are close to 1:1 in volume units but don’t represent exactly the mixing ratio used by the masons in the past since a naturally sandy lime was used. No significant compositional changes were noticed related to the building phases or the mortar function. Finally, both the binder and the binder/sand mixing ratio of a more recent mortar (presumably late 18th century) are distinctly different from the earlier mortars.