High-pressure methane sorption isotherms were measured on one Paleozoic and five Mesozoic shales, considered as targets for shale gas exploration in The Netherlands. The samples varied in mineralogy, organic richness, and thermal maturity. Four of the samples were clay-rich (total clay content 60-71 wt 96), one contained equal amounts of clays and quartz (36 wt % and 33 wt %, respectively) and one was a marl sample (clays 34 wt %, carbonates 49 wt 96). The total organic carbon contents (TOC) ranged from <1 wt 96 to 10.5 wt %, and the thermal maturity, as inferred from Rock-Eval analysis, from immature to overmature. Excess (Gibbs) sorption isotherms for methane were measured at 65 degrees C on dry samples up to 25 MPa. The maximum excess sorption capacities within this pressure range varied from 0.05 to 0.3 mmol/g (1.1-6.8 m(3) STP/t). No correlation of excess sorption capacity with TOC was found. Low-TOC, clay-rich shales had comparable or even higher methane sorption capacities per unit rock mass (mmol/g) than organic-rich shales, and a positive correlation was found between the maximum Langmuir capacity (n(L)) and the clay content. This observation supports the notion that clay minerals can contribute significantly to the sorption capacity of shales. Furthermore, we demonstrate that significant errors in TOC-normalized sorption capacities may result from the uncertainties in TOC contents, especially at low TOC values. A comparison between the immature and the overmature sample (both organic-rich with equal clay contents) did not show any enhancement of the sorption capacity with thermal maturity. However, the excess sorption isotherm of the overmature sample had a distinct maximum, while no maximum was observed for the immature sample in the experimental pressure range. A Langmuir-type absolute sorption function, with a term taking the volume of the adsorbed phase explicitly into account, gave a good representation of the measured excess sorption isotherms. The three-parameter fit yielded the Langmuir parameters (n(L) and p(L)) and a nominal density value for the adsorbed phase (rho(ads)). Two-parameter fits of n(L) and p(L) using different fixed values of rho(ads) are discussed.