Using an emissive probe, the temporal evolution of plasma potential V-p in front of an electrically isolated substrate in an asymmetric pulsed dc magnetron has been determined. The discharge pulsing frequency was 100 kHz, with a 50% duty cycle. Through a scheme of externally biasing the emissive probe, it was found that the time response of the probe could be improved greatly, and a resolution of 20 ns was achieved. This good response revealed that V-p is highly modulated by the transient cathode potential, following it closely and varying from a value just above the ground potential in the pulse, "on" phase, up to a value of +277 V during the positive overshoot in the "reverse" pulse phase. During the whole pulse cycle, V-p was found to remain above the most positive surface. in the discharge. The results confirm our previous prediction for Vp, based on energy-resolved mass spectrometry [Bradley et al., Plasma Sources Sci. Technol. 11, 165 (2002)], which indicated that ions must be created at high positive plasma potentials. However, measurements here show that the substrate floating potential V-f is also strongly modulated and the difference V-p - V-f, which determines the ion bombarding energy, always remains below 40 V during steady phases of the discharge throughout the pulse cycle. (C) 2003 American Vacuum Society.