The control of ultra-thin MoS2 film growth via annealing of thin film MoO3 precursor films was investigated. Through analysis by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy, it was demonstrated that there is significant competition between sublimation and surface diffusion, resulting in little to no grain growth at annealing temperatures of 250 degrees C-550 degrees C. At annealing temperatures above the MoO3 melting point (801 degrees C) and subsequent cooling, grain growth does occur. MoO3 decomposition products (Mo4O11 and MoO2) and a reaction product between the MoO3 film and Al2O3 substrate were also detected at annealing temperatures of 500 degrees C and above. The formation of the Al-2(MoO4)(3) phase at the film-substrate interface and the subsequent decomposition of the Al-2(MoO4)(3) producing MoO3 precursor at 850 degrees C yielded the largest grain 2D MoS2 films after sulfidation. This three step (physical vapor deposition, precursor annealing, and sulfidation) process yielded fully coalesced films of 1.5 layers thick, demonstrating the viability of high temperature annealing to obtain an interface layer that decomposes to produce MoO3 precursors resulting in very thin 2D MoS2 films.