We report on low-temperature scanning tunneling microscopy observations demonstrating that individual Ti atoms on hexagonal boron nitride dissociate and adsorb hydrogen without measurable reaction barrier. The clean and hydrogenated states of the adatoms are clearly discerned by their apparent height and their differential conductance revealing the Kondo effect upon hydrogenation. Measurements at 50K and 5×10 −11 mbar indicate a sizable hydrogenation within only 1h originating from the residual gas pressure, whereas measurements at 4.7K can be carried out for days without H 2 contamination problems. However, heating up a low-T STM to operate it at variable temperature results in very sudden hydrogenation at around 17K that correlates with a sharp peak in the total chamber pressure. From a quantitative analysis we derive the desorption energies of H 2 on the cryostat walls. We find evidence for hydrogen contamination also during Ti evaporation and propose a strategy on how to dose transition metal atoms in the cleanliest fashion. The present contribution raises awareness of hydrogenation under seemingly ideal ultra-high vacuum conditions, it quantifies the H 2 uptake by isolated transition metal atoms and its thermal desorption from the gold plated cryostat walls.