Tournai stone has been extensively used in Western European historical architecture and is subjected to severe deterioration mechanisms, which were investigated in two Romanesque portals of the cathedral of Tournai city. The stone material was studied with thin-section petrography and SEM-EDX and its hygric, hydric and thermal properties were measured. The porosity is very low and the pore size distribution can be related to either typical pores associated with the clay fraction or with micro-cracks. The formation and propagation of cracks are related to anisotropy of the stone (clay presence in laminae with a preferred direction) and the influence of different external climatic factors. As expected, the effect of liquid water has the highest damage potential while the clay fraction imparts a pronounced hygroscopic behaviour to the stone. The identified clay mineral was illite. Although its non-swelling properties exclude interlayer swelling as a cause of damage, interparticular osmotic pressure is recognized as the most plausible damage cause at the micro-scale. Gypsum crusts occur in two morphologies: films and thick deposits, both showing a comparable layered structure, but significantly differing in the proceeding process and shapes and features. Due to the nature of the stone and of the damage processes, feasible measures for preventive conservation on the long term are to be found in the direction of decreasing the climatic influence by means of sheltering.