Residual hydrogen in stainless steel results in a steady outgassing from vacuum chamber walls. hindering the achievement of ultrahigh vacuum conditions. The total content, the binding states, and the diffusivity of residual hydrogen in austenitic stainless steels, which together define the room temperature hydrogen outgassing rate, have been investigated by thermal desorption spectroscopy (TDS). Seven different steel types have been studied by means of two different TDS systems. The study has been extended to include the effects of various postproduction treatments. aimed at reducing the hydrogen content and/or outgassing, namely vacuum firing and baking both in air and under vacuum. A large variety of hydrogen desorption peaks have been observed, which have been attributed to diffusible hydrogen, hydrogen trapping in the surface oxides or in lattice defects induced by precipitates, and steel recrystallization. The hydrogen depletion effectiveness of vacuum baking at different temperatures has been quantified, and the consequences of air baking have been clarified, leading to practical guidelines for technological applications. Q 2003 American Vacuum Society.