The scanning force microscopy is used to charge insulating films locally and to monitor the decay thereafter. On Si 3 N 4 films the charge decay shows up as a discontinuous staircase, demonstrating single charge carrier resolution. The decay is controlled by thermionic emission. The smallest charge amount transferred into a PMMA film was only one or two electrons. The imaged charge area showed no broadening with time, indicating that the excess carriers are immobile.