We have studied the dipole‐allowednd3/2‐nf5/2and the dipole‐forbiddennd3/2‐ng7/2transitions of sodium in the presence of thermal ions forn=15–17. Although the cross sections for the relevant ion–Rydberg atom collisons are too small to influence the observed transitions, these microwave resonance measurements indicate that there is a remarkable influence of even very low densities, 105cm−3, of ions on the atoms. For instance, thed‐gtransition can be easily driven with a single photon, and thed‐fandd‐gtransitions are shifted and broadened. Furthermore, the thermal sodium ions destroy the interference fringes obtained with the Ramsey method of separated oscillatory fields. Although these results are incompatible with reasonable estimates of ion‐atom collision cross sections, they may be explained quantitatively in terms of the time‐averaged charge of the ions.