Hydrogen adsorption in two different metal-organic frameworks (MOFs), MOF-5 and Cu-BTC (BTC: benzene-1,3,5-tricarboxylate), with Zn2+ and Cu2+ as central metal ions, respectively, is investigated at temperatures ranging from 77 K to room temperature. The process responsible for hydrogen storage in these MOFs is pure physical adsorption with a heat of adsorption of approximately -4 kJ mol(-1). With a saturation value of 5.1 wt.-% for the hydrogen uptake at high pressures and 77 K, MOF-5 shows the highest storage capacity ever reported for crystalline microporous materials. However, at low pressures Cu-BTC shows a higher hydrogen uptake than MOF-5, making Cu-based MOFs more promising candidates for potential storage materials. Furthermore, the hydrogen uptake is correlated with the specific surface area for crystalline microporous materials, as shown for MOFs and zeolites.