These files contain the data supporting all of the results in Katie Gann, et. al, Initial nucleation of metastable γ -Ga2O3 during sub-millisecond thermal anneals of amorphous Ga2O3. In this work, we found: Beta-phase gallium oxide (β-Ga2O¬3) is a promising semiconductor for high frequency, high temperature, and high voltage applications. In addition to the β-phase, numerous other polymorphs also exist and understanding the competition between phases is critical to control practical devices. The phase formation sequence of Ga2O3, starting from amorphous thin films, was determined using lateral-gradient laser spike annealing at peak temperatures of 500-1400 °C on 400 μs to 10 ms timescales, with transformations characterized by optical microscopy, X-ray diffraction, and transmission electron microscopy (TEM). The resulting phase processing map showed the γ-phase, a defect-spinel structure, first nucleating under all annealing times for temperatures from 650-800 °C. Cross-sectional TEM at the onset of γ-phase formation showed nucleation near the film center with no evidence of heterogeneous nucleation at the interfaces. For temperatures above 850 °C, the thermodynamically stable β-phase was observed. For anneals of 1-4 ms, and for all anneals below 1200 °C, small randomly oriented grains were observed. Large grains were observed for anneals below 1 ms and above 1200 °C, with textured films resulting from anneals above 4 ms and 1200 °C. The formation of the γ-phase prior to β-phase, coupled with the observed grain structure, suggests that the γ-phase is kinetically preferred during thermal annealing of amorphous films, with β-phase formation subsequently forming by nucleation at higher temperatures. The low surface energy of the γ-phase implied by these results suggests an explanation for the widely observed γ-phase inclusions in β-phase Ga2O3 films grown by a variety of synthesis methods.