The polarization of assembled electret microphones with energetic electrons from a Cockcroft‐Walton accelerator is discussed. Electron transmission curves of metallic foils and of the dielectric films Mylar and FEP Teflon are given based on experimental measurements. Results of charging experiments on Mylar and FEP are presented which show how the optimum electron bombarding energy and beam intensity were chosen for polarization of a given microphone. The method devised to monitor microphone performance in vacuum while polarization is carried out is described. The polarization of a specific microphone is discussed to illustrate the use of the electron transmission data and the method of monitoring the microphone performance.