Abstract The aim of the present study was to investigate the porosity of the Posidonia shale as a potential gas shale and to compare the results with data published on gas shales known to be productive. The characterization of the porosity of clays and shales still poses an analytical challenge, however. Different methods were investigated based on a comparison of four different Posidonia shales, with different degrees of maturity. Both direct microscopical methods as well as indirect methods based on gas adsorption or Hg intrusion were applied. Most of the pores in clays and shales were too small to be detected by any of the existing direct methods. About 80% of the pores were <30 nm wide. The Posidonia shales, as is the case with most shales, are dominated by mesoporosity (ranging from 20 to 50 mm 3 /g). The mesopore peak representing the average pore diameters could be resolved by Hg intrusion and was found to decrease with increasing maturity which may be explained by increased compaction and/or temperature. This relation, if not applicable to the entire Posidonia shale, may be restricted to a single sedimentary system (e.g. a basin or a sequence). The results of the indirect methods (except for CO 2 microprosity) were comparable and this might be explained by the low macroporosity. The most important question concerning shale-gas production and porosity relates to the pore diameter needed to allow gas migration. Natural gas in micropores may be bound too strongly to liberate it without low pressure/vacuum. For gas production either meso- or macropores may be important. Notably, all methods used for characterization of porosity were performed on dry samples. A nm-scale connectivity determined at an illite-smectite interface pore is not supposed to provide a gas pathway in the water-saturated state. Prediction of the gas-production potential from porosity measurements has not been possible to date, therefore. Taking the organic carbon content, vitrinite reflectance, and comparison of the porosity with that of North American gas shales into account, however, indicates potential for the Posidonia shale which still has to be proven by shale-gas production tests.