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Thin germanium films on SiO2 completely melted by pulsed laser irradiation cool rapidly by thermal conduction to the substrate until they solidify. In situ measurements indicate that the liquid is supercooled by 420–530 K with respect to the crystalline phase prior to solidification. Cross-sectional transmission electron microscopy reveals nucleation events at the Ge/SiO2 interface. The microstructure of these events is comprised of a very fine grained (5–15 nm) polycrystalline core with much larger grains extending laterally and toward the free surface. It is believed that nucleation of the amorphous phase, which was subsequently converted to the fine-grained material, initiated solidification.