Four maps, based on Conodont Colour Alteration Indices (C.A.I.s) have been established for Belgium : one for the Eifelian and the Givetian, one for the Frasnian, another for the Famennian and a last one for the Tournaisian and Visean. Conodonts have not been found in the Cambrian of the Ardennes and as they are scarce in the Ordovician-Lower Devonian of this area, it was impossible to integrate these C.A.I. data into an isograd map. The maps for the Middle Devonian-Lower Carboniferous interval show variations in C.A.I. which can be explained by sedimentary burial and by regional Varistic deformation patterns. In contrast to the Ardennes where in general conodonts are showing C.A.I. values of 3.0-5.0, indices in two adjacent areas, the Eifel Hills (Germany) and the Boulonnais area (France) are ranging between 1.5-2.0.