We present a model for the formation of silicon nitride on monocrystalline Si(1 0 0)-(2 x 1) at 300 K using gaseous ammonia (NH3) as nitridation reagent. FT-IR was used to identify surface species. From crystallographic and thermodynamic considerations the model shows the different steps of formation of Si3N4 and the chemical species formed during this process. The model is in full agreement with literature data obtained by various physico-chemical techniques. The model explains what happens at high temperatures on the atomic scale and allows us to derive some important conclusions about the last steps of nitridation. (C) 2002 Published by Elsevier Science B.V.