Tin nanocrystals embedded in a SiSn layer grown by molecular beam epitaxy on n-type Si are investigated by means of deep level transient spectroscopy. Two Sn related deep traps are observed, depending on the annealing temperature of the samples. A deep level at [Formula: see text] (Sn1) is observed for annealing temperatures up to [Formula: see text]C, whereas a level at [Formula: see text] (Sn2) appears for annealing temperatures above [Formula: see text]C. Scanning transmission electron microscopy shows the formation of Sn nanocrystals at [Formula: see text]C, which coincides with the appearance of Sn2. Sn1 is tentatively assigned to a Sn related precursor defect, which transforms upon annealing into either Sn nanocrystals or an interface defect located at the nanocrystal surface.