This paper describes charge trapping, detrapping and memory cycling and retention in ultra-thin oxide-nitride-oxide (ONO) structures in submicron FET devices with polysilicon gates, also known as silicon-oxide-nitride-oxide-silicon (SONOS) structures. The ONO films had various thicknesses (4.9-9.0 nm) and their top oxide was obtained either by reoxidation of nitride or by in situ deposition of oxide by chemical-vapor-deposition (CVD). Memory characteristics are found to be critically dependent on details of processing and film thicknesses. The reoxidized ONO films show larger threshold shifts than the CVD films. A surprising result is that the "write" threshold shift could be "erased" only in one of the reoxidized structures. © 1991.