We present a simple yet absolute method for the assignment of the zero charge state of metal nanoparticle films during their electrochemical charging. The method is based on measuring the current transient that accompanies the potential controlled immersion of a pristine, ion-free cluster film in an electrolyte, and does not rely on relative quantities such as mass changes of an already immersed film. Therefore, the method is impervious to details of counter ion insertion - for instance whether the electrolyte can partition into the film or not - without which no charging can be observed. Examples of positive and negative ion-limited charging are presented, and an example of ambipolar cluster charging, covering the three possible cases. The immersion method can be readily applied to other cases of coupled electron-ion transfer, as in the three-phase electrochemistry of redox droplets and thin films. © 2012 Elsevier B.V.