A high pressure chamber (HPC) has been developed in collaboration with E. Bauer (Arizona State University) and M.S. Altman (Hong Kong University of Science and Technology) is available as ancillary chamber for the SPELEEM microscope. The HPC can operate at mbar pressures, carring out mass spectrometry TAP (temporal analysis of reaction products). This is possible thanks to a double differential stage for pumping the quadrupole mass spectrometer. We have successfully commissioned the HPC in fall 2012. In spring 2013 the chamber will be used to study the reactivity of nano-sized metal clusters supported on oxide thin films. The SPELEEM will be used to characterize the surface before and after chemical reactions.