This letter describes a technique for measuring the ion current at a semiconductor wafer that is undergoing plasma processing. The technique relies on external measurements of the radio-frequency (rf) current and voltage at the wafer electrode. The rf signals are generated by the rf bias power which is normally applied to wafers during processing. There is no need for any probe inserted into the plasma or for any additional power supplies which might perturb the plasma. To test the technique, comparisons were made with dc measurements of ion current at a bare aluminum electrode, for argon discharges at 1.33 Pa, ion current densities of1.3–13 mA/cm2,rf bias frequencies of 0.1–10 MHz, and rf bias voltages from 1 to 200 V. Additional tests showed that ion current measurements could be obtained by the rf technique even when electrically insulating wafers were placed on the electrode and when an insulating layer was deposited on the electrode.