In the quest to accelerate the discovery and deployment of new materials with ideal and tailored properties, synthesis via solid-state diffusion holds particular promise as it allows the crystal structure and stoichiometry of thin films to be controlled independently of the deposition method. However, the kinetics and the quality of the resulting materials remain relatively unexplored. Here we demonstrate both source-limited and kinetically-limited solid-state diffusion as routes to tune the stoichiometry of platinum silicide (PtxSi) thin films, representative of metal silicides which have attractive mechanical and electronic properties. Using in situ heating inside a transmission electron microscope (TEM) while performing electron diffraction, we show that both routes lead to stoichiometrically-controlled formation of PtxSi (x = 1, 2, 3) thin films with high phase selectivity, revealing the crystal structure and formation sequence for each phase. The PtxSi formation process from sequentially-deposited layers of platinum (Pt) and amorphous silicon (a-Si) significantly differs from the Pt/single-crystal silicon (sc-Si) case, allowing the formation of Pt3Si thin films for the first time.