The effects of nitrided SiO2/Si(100) interfaces upon cycling endurance in silicon–oxide–nitride–oxide–silicon (SONOS) nonvolatile memory transistors are investigated. Analysis of metal–oxide–silicon field-effect transistor subthreshold characteristics indicate cycling degradation to be a manifestation of interface trap generation at the tunnel oxide/silicon interface. After 106 write/erase cycles, SONOS film stacks prepared with nitrided tunnel oxides exhibit enhanced cycling endurance over stacks prepared with non-nitrided tunnel oxides. If the capping oxide is formed by steam oxidation, rather than by deposition, SONOS stacks prepared with non-nitrided tunnel oxides exhibit endurance characteristics similar to stacks with nitrided tunnel oxides. For this case, a mechanism for latent nitridation of the tunnel oxide/silicon interface is proposed. © 1999 American Institute of Physics.