The influence of solute oxygen and nitrogen on the magnetic breakdown and residual losses of superconducting niobium cavities was investigated experimentally by exposing the cavities to these gases at pressures up to 10−5Torr during the heat treatment at 2100 K. We have collected evidence that the oxygen exposure increases the residual losses and, at bulk concentrations of about 0.1 at.%, the breakdown field is substantially lowered. Penetration depth measurements seem to indicate high values of the Ginzburg‐Landau parameter &kgr; ≈ 4.4 in the surface sheath which could explain the low rf breakdown observed.