Thermal desorption spectroscopy (TDS) utilising a quadrupolar mass spectrometer is a highly sensitive and selective method to study the hydrogen desorption of hydrogen storage materials. For a quantitative analysis of the hydrogen storage capacity, the TDS apparatus can be consistently calibrated by using hydrogenated PdGd alloys or TiH2 as standards. Owing to its hygroscopic nature, the use of CaH2 as calibration standard leads to erroneous results. The chemical reactions during the thermal desorption of CaH2 are analysed in detail. (C) 2003 Elsevier Science B.V. All rights reserved.