A corona-charging method for obtaining island charge distribution electrets has been described. Two series of polymer electrets–with uniform and island surface charge distribution–have been investigated. The electrets have been made from 20 μm thick PTFE and PP foils and have been stored in a desiccator at room temperature and high humidity of about 98%. Two series of PP samples have been stored at an elevated temperature of 72 °C. The dependence of electret surface potential on storage time shows that at low temperature and high humidity the island charge distribution electrets are more stable.