Masonry mortars and mortars used for flooring (4th-11th century AD) from excavations in the Cathedral at Tournai in Belgium have been investigated as part of a broader study considering the transition of ancient cities from Roman to late Roman society and the transformation to the early middle ages. A selected set of samples has been characterised using a combination of chemical and microscopical techniques. This characterisation enables us to refine our knowledge of the mortar composition, of the original materials and of their provenance. The results of the characterisation of these samples clearly indicate the importance of an optical microscopy study using thin sections as a first step in the chemical-mineralogical characterisation of historic mortars because of the complexity and heterogeneity of this composite material. Microprobe analysis results on lime lumps have clearly proved to be useful to determine the hydraulicity of ancient mortars. (C) 2004 Elsevier Inc. All rights reserved.