This paper describes an ultra-high-vacuum thermal desorption spectrometer, designed to study hydrogen in steels at ∼1 wt ppm (∼50 atomic ppm) concentration. The high sensitivity achieved also facilitates the analysis of surface phenomena. The instrument was evaluated with model materials and provided good measurements of diffusible hydrogen. A hydrogen peak at ∼350 °C was identified for steels exposed to water during hydrogen charging, and attributed to water molecules adsorbed on the sample surface for samples exposed to the laboratory atmosphere for times as short as 1 min. Recommendations are made for precautions to be taken when handling the samples.