Fair-faced concrete does not only have a technical function to fulfil but also an esthetical one. The appearance of such a concrete surface depends on building site conditions as well as on materials and production methods of the concrete itself. Within the framework of a research on the appearance and colour acceptation of different finishing building materials, funded by the Belgian FPS Economy, a case of discoloured fair-faced concrete was studied by means of petrography. As colour and texture differences of finishing materials have major impact on the human acceptance, understanding the phenomenon of discoloration could lead to controlling the parameters of influence in the production phase and as such deliver good quality concrete in terms of performance and appearance. A petrographic examination combined with ESEM-EDS analyses showed that the observed dark discolouration has its origin in the concrete microstructure. Our findings fit not fully the model, proposed by D. Strehein and P. Schiess (2008), concerning the dark mottled discoloration as the impact of weather conditions can be neglected in this case study. But the same transport and crystallization process has occurred although probably triggered by an inaccurate action during the compaction of one of the loads of fresh concrete e.g. vibrating to long. This could have led to the expulsion of capillary water which caused calcium hydroxide to deposit on the surface.