Most superconducting rf cavities consist on Nb and most Josephson tunnel junctions are Nb based. Due to the very small dissipation in these superconducting cavities and tunnel junctions quantitative and qualitative new surface effects emerge, which are related to the oxidation of Nb. The oxidation of Nb is dominated by the solution of oxygen in Nb and the growth of Nb2O5 microcrystallites. Both effect heavily strain the Nb-Nb2O5 interface creating O segregates and serrating the interface, growing with oxidation. These effects yield a serrated and eroded transition superconductor — insulator, which affects the properties of superconducting rf cavities and tunnel junctions significantly.