The electron energy distribution function (EEDF) has been measured under a variety of conditions in an Ar/Cu plasma for ionized physical vapor deposition. The EEDF is directly measured in a system including a direct-current magnetron sputter source for copper and a radio frequency (rf) induction plasma, using a Langmuir probe with a modulated bias voltage in combination with a lock-in amplifier. The experimental data indicate that at fixed rf ionization power, the electron population in the tail of the EEDF is depleted by the introduction of copper vapor, and the electron average energy decreases slightly. Observed changes in the EEDF are attributed to inelastic collisions with copper atoms, which have lower threshold energies for excitation and ionization as well as larger cross sections as compared to argon, and the resulting reduction in the measured plasma potential.