A study of outgassing by impacting surfaces in ultra high vacuum using mass spectrometry is presented. The substrate, which receives the impact, is a stainless steel AISI 304 sheet. The impacting surface is a carbon chromium ball. Impacts were performed at energies of 3.36 and 9.8mJ. The average pressure increases over 20 impacts for the first desorption peak in the ultra high vacuum system were 1.0x10 - 9 and 1.9x10 - 1 0 mbar for the high and low energy impacts, respectively. Hydrogen was the main desorbed species with traces of methane. A significant reduction of the pressure intensity increase was always observed after the first impact. Experiments performed by exposing the sample surface to deuterated water at a pressure of 6x10 - 8 mbar for 10min did not show any D 2 O, DHO, DO, D 2 or O 2 desorption. Optical microscopy and atomic force microscopy of the surface subject to impact revealed regions of intense plastic deformation as well as the occurrence of craters. We conclude from these experiments that desorption under mechanical impact in ultra high vacuum comes from the release of hydrogen dissolved in the bulk of the AISI 304 steel substrate. The contribution of adsorbed species to outgassing is negligible.