International audience Permian Rotliegende sandstone cores were collected from an area of about 14,800 km2 (!5710 mi2) mostly to the eastsoutheast of the city of Bremen in northern Germany, at depths between 4596 and 5330 m (15,079-17,487 ft). The separated size fractions (<0.2, 0.2-1.0, and 1.0-2.0 mm) consist of illite (90-100%)with small to minute amounts of chlorite and detectable quartz and feldspars in the coarser fractions. Scanning electron microscopic and transmission electron microscopic observations showed two types of illite morphologies: flakes coating detrital framework minerals and laths and fibers invading the pore space. The data points of most size fractions fit two isochrons, with slopes providing ages of 191 ± 8 and 178 ± 1 Ma, with initial 40Ar/36Ar ratios reasonably close to the atmospheric value. Microthermometric fluid-inclusion determinations in quartz and calcite characterize two types of percolating fluids: a highly saline (19% NaCl equivalent) fluid at variable temperatures depending on the reservoirs (185 to 150°C) and a slightly saline one (2.6% NaCl equivalent) again at varied temperatures (170 to 145°C), also depending on the locations. These temperatures are higher than paleotemperatures calculated on the basis of a present-day burial gradient of 30.5°C/km, therefore favoring hydrothermal illitization, with the oldest illite crystallizing at a generally higher temperature than that of the younger illite.