The Big Chamber has a long history. It is equipped with two differentially pumped quadrupole mass spectrometers (QMS). One is primarily used for measuring the desorption of small species, particularly hydrogen, whilst the other can detect species with masses up to 500 amu. The latter QMS can also be rotated to facilitate angle resolved studies. A rotatable ion detector and laser access window provide the means with which to perform resonance enhanced multiphoton ionization (REMPI) measurements to investigate the internal energy of desorbing species. The chamber is also equipped with several viewports that can be used to expose samples to a range of light sources, as well as a hot capillary atomic hydrogen source and a molecular doser. Single crystal surfaces can be cleaned and assessed with the standard combination of noble gas ion sputtering / annealing cycles, low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The sample can be mounted on either a water/liquid nitrogen cooled holder or an alternative liquid helium flow system.