Some of the key parameters used in process simulators are formation energy, diffusivity and the concentration of Si self-interstitials. Unfortunately, experimental verification of these parameters is lacking. This work is therefore designed to determine the equilibrium concentration of self-interstitials at various temperatures and thus compute the formation energy for selfinterstitials in Si. Samples with thin (10 nm) buried boron layers were grown by MBE and samples with a buried type II dislocation loop layer were produced by Ge+ implantation into undoped MBE. These samples were subjected to a 40 KeV 1E14 /cm2 Si+ implant followed by anneals at temperatures between 685°C and 815°C for varying times. The loop layer was investigated to monitor the total flux of the interstitials as a function of time while the broadening of the B layer profile was analyzed by SIMS to determine the interstitial supersaturation. A combination of these two values provides an estimate of the equilibrium concentration of the Si interstitials. The results at various temperatures are then used to extract the enthalpy of formation of the Si interstitial.