The plasma potential and the potential of an electrically isolated surface are measured in an rf diode sputtering glow discharge. The influence on these potentials of both the geometry enclosing the discharge volume and of a positively biased auxiliary electrode in contact with the discharge is investigated. It is shown that confining the discharge increases the plasma potential and the energy of positive ions incident on electrically isolated substrates, whereas applying a positive voltage to an auxiliary electrode also increases the plasma potential but does not significantly increase the energy of ions incident on electrically isolated substrates. The effect of rf modulation on the ionic energy distributions is demonstrated. This occurs as the ions pass through the plasma-substrate sheath and results in a large broadening of the energy distributions of low-mass species. © 1972 The American Institute of Physics.