Collecting and emitting probes have been used to explore the distribution of potentials and the dynamic behavior of the cathodic sheath in the rf glow discharge plasmas used for plasma immersion ion implantation (PI3). On application Of a negative voltage pulse, the sheath expands rapidly but comes to a steady-state position within a few microseconds. The potential distribution then remains stable as long as there is sufficient plasma outside the sheath region to replace the ions lost to the cathode. The various dc potentials in the plasma that arise from rectification of the rf fields are also important. At power levels up to 500 W, the plasma potential is several hundred volts above the average dc potential of the antenna. The magnitude of this voltage difference depends on the antenna geometry, its insulation from the plasma and the dc potential that can exist between it and the chamber walls. The floating potential of a typical pI3 target can vary from 50 to 300 V below the plasma potential but in all cases the difference is sufficient to ensure that there is a sheath and presheath structure around the target before the application of the high voltage pulse.