  In this paper the interaction of hydrogen molecules with atomic-sized
superconducting nanojunctions is studied. It is demonstrated by conductance
histogram measurements that the superconductors niobium, tantalum and aluminum
show a strong interaction with hydrogen, whereas for lead a slight interaction
is observed, and for tin and indium no significant interaction is detectable.
For Nb, Ta and Pb subgap method is applied to determine the transmission
eigenvalues of the nanojunctions in hydrogen environment. It is shown, that in
Nb and Ta the mechanical behavior of the junction is spectacularly influenced
by hydrogen reflected by extremely long conductance traces, but the electronic
properties based on the transmission eigenvalues are similar to those of pure
junctions. Evidences for the formation of a single-molecule bridge between the
electrodes -- as in recently studied platinum hydrogen system -- were not
found.
